I'm gonna give my answer based off of experience.
Each boss or dungeon (not sure) has unique items, how many unique items I couldn't tell you, that drop from bosses. They also drop the generic gear, for example "Leather belt of stamina". You are not always garunteed to get good items from a boss.
I have gotten everything from nothing from an entire dungeon to a couple epics in a dungeon, it is purely luck.
Also something to note, every elite boss in ESO has a unique item. Atleast that I have come across.
Long story short, yes you can make 50 on just PvE.
I am currently half way through Veteran Rank 1 and I had no problem getting to level 50 with just quests. It was close I will say, being on the final few quests before finishing coldharbour. I will also mention that a lot of people are having issues with finishing coldharbour and only being level 45-49.
I pvpd maybe once during my journey to 50. I did the beginning questline that gives away 2 skillpoints, and then continued to finish the 20 kills quest. so maybe a small 30-40k boost of experience, so nothing huge. I did every quest I could find, only not finishing Shadowfen (by 7 quests) and The Rift (by one quest) because I couldn't find them. Also missed one quest in Coldharbour because it is bugged (I keep going back to it every morning as this seems to fix most bugged quests, I also wake up at 7 am everyday so it is pretty quiet and I think the servers don't freak out as much.) I know I missed these quests because the achievements tell me so, although I have noticed in Deshaan and Stonefall had more quests then the achievement said there were.
I would say a lot of peoples issue with hitting level 50 on times is that they only do 70% of the content. As Exploring, public dungeons, private dungeons, and side quests is atleast 50% of what you need to level on time. While the main storyline quests and the quests you happen to walk by doing the main storyline is the other 50%. So if you are worried about not making 50 on time make sure you are doing atleast 90 - 95% of everything so that you don't find yourself spamming coldharbour for a grind party.
I will finish by saying I also found myself multiple times 1-2 levels below the mobs and quests, it was confusing me a little at first but do not worry. Take your time, enjoy Tamriel and remember to take some time to just explore, discover and finish some things outside the beaten path.
Best Answer
Disguises and costumes (as well as the upcoming guild tabbards) are all cosmetics only (i.e. they don't have any influence on your stats), with slightly different behavior:
Note that the naming ingame isn't very consistent. Most quest disguises will disappear once you're hit in combat, while some of those are permanent (e.g. the disguise obtained on Stros M'Kai).