Hiya, I only played a priest until level 45, so I can give you answers to my experience up until that point. I was pure holy spec and also relatively new to the game. I also had a 64 tauren arms warrior, if you want any information on that.
1) Hard to kill? No, in my experience. To be honest my only real defense was bubbling and healing. I have heard however that Disc/Holy priests are very difficult to kill, pain in the ass.
2) Highly needed/sought out in groups? Very much so, I found myself adding alot of people to buddy, and constantly being asked to heal. To be honest though I think that during lower levelled dungeons/instances a druid could also do this job. You really need to enjoy healing to enjoy being a holy priest, either that or you need to really enjoy group wipes.
3) Capable solo questing? Yes but it was very slow, I mana burned alot after having allocated no points to disc.
4) Easy to find appropriate gear for throughout the game, particularly instance boss drops? Yes relatively, they'd always have something +spell + healing or +spell crit, if not tailoring is always an option as cloth is a regular drop, and it's quite an easy profession to up.
5) Do trolls make good priests? I'm not sure about this one, sorry.
6) Viable in PvP? I believe that disc or shadow specced priests would be better for this, however during my low level PVP's, it was more successful to be doing BG's like Arathi Basin as opposed to WSG, easier to offer heals to people without being one-on-oned completely as well as give buffs and dispel from a distance.
7) Most importantly, fun to play? For me it was entertaining, the only thing that disappointed me was questing, but if you have friends to play with commonly, then you're going to level as quickly as everyone else.
