You can use (almost) all of your powers when disarmed and without implements.
ImplementDDI and WeaponDDI powers don't prevent you from using them if you lack the appropriate tool: they simply allow you to get a bonus when you have the appropriate tool.
When using an Implement (keyword) power you make an unmodified ability check (usually Wisdom-based for Implement Cleric powers) against the target defense (as per definition of attack roll).
When you hold a magical implement (e.g.: a holy symbol), you get to add the implement's enhancement bonus to both attack rolls and damage rolls with Implement powers (and a damage boost in case of critical hit).
Also note that you can take advantage from any implement you are proficient with (not only holy symbols). If your Cleric took the Arcane InitiateDDI feat, then he can use even magical staves, wands and orbs to improve his Cleric powers.
A non-magical implement does not provide any bonus (except for flavor or feat support) unless it is a superior implement. Superior implements cost more but provides a varied array of advantages (properties similar to those of weapons).
For example, the Astral SymbolDDI costs 18 gp but has the Distant (+2 range of ranged and area powers) and Energized (radiant) (+2/+3/+4 damage bonus with Radiant powers) properties. Its base version is non-magical, but it you can buy or create an enchanted version of it.
You gain proficiency with a single type of superior implement by taking the Superior Implement TrainingDDI feat.
Lack of proficiency with an implement you are using means that you gain no advantage from its superior or magical bonus and properties.
When using a Weapon (keyword) power while unarmed, you make an unmodified ability check (usually Strength-based for Weapon Cleric powers) against the target defense (usually AC).
However, unarmed attacks have no proficiency bonus, so you lack a thick +2 or +3 modifier.
When you wield a non-improvised weapon you get a proficiency bonus to attack rolls with Weapon powers.
When you wield a magical weapon, you also get the weapon's enchantment bonus to attack rolls and damage rolls with Weapon powers (and a damage boost in case of critical hit).
Lack of proficiency with a weapon you are wielding means that you don't apply the weapon's proficiency bonus and gain no advantage from its magical bonus and properties.
Some powers may require you to wield specific weapons or implements. In this case, they explicitly state this requirement in their Requirement line of the power block. This is usually found in Rogue powers, for example.
Best Answer
To sum up: Buy the hilariously underpriced ring of evasion and save boosting and nondetection for a party member who will benefit more for as much as they're willing to spend on it. If you can't pool funds or have no high-dex/reflex sneaky characters who would benefit, get the much more generic periapt of wisdom
Let's look at this from an abstract perspective first:
A periapt of wisdom is worth 4000 gp.
A ring of evasion is worth 25,000 gp + a ring of nondetection (less the immunity to detect thoughts , so we'll price it as a custom item) of 3*2*2000*1.5 (because it's extra) or 18000 gp (which is amazingly overpriced compared to the ring of mind shielding + a ring of fort +2 estimated at 4000 gp * 1.5 * 1.5 (fortitude boosts don't go on the ring slot.) or 9000 gp for a grand total of 52000 gp.
Thus, if you have reason to suspect that a periapt of wisdom will be available more cheaply in the future, this will inform your choice.
However. To a cleric, wisdom is all. Wisdom is all. Evasion only works if you pass a reflex save. At level 7 with a 10 dex, you have a wonderful +2 to your reflex. (selling this ring to your thief-like character on the other hand for improved evasion should allow you to arbitrage a rather inefficient market.)
+2 to fortitude is your only "real" bonus from the ring, considering how useful the rest of the suite is to your specific capabilities. (again, if you can sell it to other characters, do so for as much as they're willing to spend up to 52,000 gp).
Looking at bonus spells per day, the tradeoff is this. How often is your character targetted by save-or-lose fortitude effects versus how much would you like a +1 dc to all spells and an extra 4th level spell per day and a bonus +1 to your will save?
The answer will vary by campaign, but it would be hard to be more persuasive by the wisdom boost, save if the cost was such that market arbitrage could provide for the same bonus, later, for less expenditure.