I researched so much, I've decided to just post what I've noted as an answer.
The following data were collected according to as many screenshots as I could capture from the $10K Alienware Arena Cup (which took place 19 April 2014, which is important because the developers change items often). Lesser-known teams and names were given less weight. Items are usually enumerated in a meaningful order.
- As with any other game-mode, Tabi are staples for physical characters and Shoes for magical characters.
- Items that build stacks are discouraged.
- Common actives are Heavenly Agility, Greater Sprint, Curses, Greater Aegis, Greater Purification Beads, and Greater Blink.
- Magi's Blessing was common for all roles.
- Hide of the Urchin is popular for anyone but Mages.
- Physical characters usually get Warrior Tabi, Jotunn's Wrath, and Titan's Bane. Other items (in descending order of popularity) include Deathbringer, Rage, Qin's Sais, Shifter's Shield, and Void Shield.
- Warriors (bruisers) don't pick purely offensive items other than Jotunn's Wrath, Titan's Bane, and Qin's Sais (Guan Yu gets not even this).
- As a sort of response to Rapitor's answer, I checked how many times Ninja Tabi appeared and noted only three instances: Anatoliy as Sun Wukong in round 3 of Dignitas vs. COG Gaming, GameHunter as Tyr in round 3 of TSM vs. Exposed, and InfernalTwist as Sun Wukong in the first round of Rage vs. Dignitas. Otherwise, Warrior Tabi was picked by an overwhelming majority.
- Mages usually get Shoes of the Magi, Rod of Tahuti, and Obsidian Shard. They often get defensive actives, like Aegis, Beads, and Combat Blink, but sometimes Heavenly Agility or Greater Sprint. Other items (in descending order of popularity) include Polynomicon, Shoes of Focus, Breastplate of Valor, Spear of the Magus, Bancroft's Talon, Chronos' Pendant, Void Stone, and Gem of Isolation (seen on Chang'e, Poseidon, and Ao Kuang.
- Guardians (tanks) usually get Sovereignty, Shoes of Focus, and Shell of Absorption.
- Chronos commonly gets Hastened Fatalis and less commonly Demonic Grip.
- Freya commonly gets Hastened Fatalis and Demonic Grip.
- Vamana commonly gets Hastened Fatalis.
- Demonic Grip is common with Freya but less common on Chronos, Poseidon, and Nu Wa (small sample size).
- Top Arena League player CeeVee usually gets Rod of Asclepius and Heavenly Agility as Chang'e.
- Greater Blink is common with Ares, Athena, Bacchus, Chaac, Geb, Hun Batz, Kumbhakarna, Odin, and Tyr.
- Be mindful that some items may be good with other characters unmentioned, but I haven't mentioned them because the sample that I studied (Alienware Arena Cup teams) did not pick those characters, which hints at which characters that you think would go good with an item that I mentioned are not good characters for Arena.
- A new item to consider is Ancile for physical characters, though it is a stacking item (+5/+1, 60). It maxes out at +50 Physical Power and +15% Cooldown Reduction (and +50 Magical Protection).
Some URLs redundantly appear as multiple sources in the code. If someone could clean them or teach me how to (efficiently), I'd appreciate it.
It sounds like you're trying to be the "star of the show". I highly recommend not jungling. Early game success is mostly about team coordination.
At the start of the game your team has a choice. To invade, or go for buffs. Usually buffs are the better choice. Once buffs are collected, go to the lane and help your team with minions while poking the enemy gods. If you have a good team combo, you could pull off an early game deicide. Just be careful of their team combo.
If you're team dies, and you're in the jungle collecting buffs, you could be blamed, depending on how fed your team is, the gods you are all playing and the reason of their death. This would most likely be a team tower dive, in which case, you are too blame.
Early game does not revolve around getting kills, and if you die once early game, you can recover by working as a team and getting a kill. You don't need to be the one to get the kill. Smite is a game based around teamwork. A kill is a kill, no matter who gets it.
Personally, I find Loki to be quite a bad joust god. Try playing Thor. Try gathering some friends and coming up with a team who can combine ultimates. For example, Hades, Hou Yi and Thor. Thor can go into jungle and collect a buff, then ult. Hades and Hou Yi ult, and Thor executes the last bit of his ult, securing the kill.
Hope this helps, good luck!
P.S. I love loki as a character, but he performs better in Conquest or Arena.
Best Answer
General Guidelines
Usually, magical gods that are looking to optimize their healing will prioritize power, MP5, and cooldown reduction in some way. Power will help determine how strong your heals are, cooldown reduction will help determine how quickly you can press your heal button again, and MP5 will help determine how often you can press the heal button until you run out of mana. There is hard cap on cooldown reduction of 40%, and while there is also a cap on MP5 I believe, it isn't usually particularly relevant because it's pretty high and if you are going to reach it, you'll do so on accident.
To this point, the first item most players that are prioritizing healing pick is Chronos' Pendant. It provides a healthy 75 power, but more importantly it provides 25 MP5 and 20% cooldown reduction. I typically play Assault mode, and this is almost always the first item any mage-type god takes, but there's no exceptions I know of when it comes to mage healers.
Healer-focused items
There are two note-worthy items that enhance the abilities of a healing mage. The first is the Rod of Asclepius, which provides 75 power, 250 health, and 10% movement speed (which is quite great for Ra, but useful in general). The really unique thing about this item is the aura it grants, which increases the healing and health regeneration of allies within 70 units by 25%.
The second item, which may be more situational for certain gods, is called Lotus Crown. It provides 60 power, 20 physical protection, and 20 MP5. It also has a passive ability that grants a +20 physical and magical protection buff for 5 seconds on any god affected by your heal abilities, although it cannot activate on the same target within 10 seconds. This is really good for healing mages that heal in a radius around themselves, since it means they'd have to be near danger to heal in combat. On Ra, this is more situational, but still useful, especially if you could put the physical protection or MP5 it grants to good use.
Boots
The only two boots I get as a healer are Shoes of the Magi (45 power, 10 magical penetration), or Shoes of Focus (30 power, 250 mana, 10% cooldown). I decide which to buy based on the enemy team composition. If there are a lot of physical champs on the opposing team, then I'll buy the Shoes of the Magi. Even though it lacks the cooldown reduction that the Shoes of Focus grants, I'll get Shoes of the Magi because I know that I'll get a physical defensive item later on, and there's a great option that comes with a lot of cooldown reduction (see below).
Late-game items
From there, the rest of your items will be highly situational and depend on the specific environment of your game. More power and cooldown reduction will help your heals. But you also need to be able to survive your lane and team fights. As an assault player, I can tell you that, depending on the enemy team comp, I will often pick up a physical defensive item (Breastplate of Valor for it's 75 physical protections and 20% cooldown reduction), or a magical defensive item (Pestilence for it's anti-healing aura for your opponents or Bulwark of Hope for additional survivability).
Then, I'll generally prioritize either power or cooldown reduction. If I have Chronos' Pendant and Breastplate of Valor, then I'm capped on cooldown reduction and will focus on power, with Rod of Tahuti being a prime choice if I can finish buying it in a reasonable amount of time. If I don't have the breastplate, then I might look at Pythagorem's Piece for the extra 10% cooldown reduction it provides if I still feel the need to cap it.
I'll probably also invest in magical penetration, although that's not always a given. Be aware of what the enemy team is building themselves. If some enemy players are stacking some magical defense items, then look into getting a Obsidian Shard, which allows you to penetrate 33% of your target's magical defenses, or some other penetration item you'd prefer.
There's some other situational items as well, depending on circumstances, that you might want. Divine Ruin is a great item if enemy healers are giving your team trouble. Warlock's Sash is nice if it's available in your game mode, and a good case can be made to picking it up early in the game. Soul Reaver can help damage those high-hitpoint targets. None of this really helps with healing specifically, but Ra can also be a pretty powerful offensive god with great range. Once you've got your core healing items, don't forget that you also have to deal damage in team fights too, and these items still provide healthy amounts of power while providing you with more utility to help your team.
Note: While you're asking specifically for Ra, a lot of this answer can be applied to other healing gods. However, Guan Yu is a physical god that is considered a healer. Almost none of these items apply to him, although some of the strategies do.