To answer your question, Beastmaster is the solo tank that requires the least amount of money. Beastmaster has lower damage than the two, but will not die because it has built in defense and heals. Definitely the easiest to pick up, will find parties pretty easily if needed to, build is not complex whatsoever. You do not need to refine your gear or get heal pets if you don't want to. You could even venture into PVP environments if you get bored and you'd do pretty well. If you feel like learning more, hop on over to the Beastmaster subforums.
Priest prefers (maybe requires) refinement to achieve the high defense, but have the heals. Warriors have the defense but need the heal pet to survive long fights. I personally know players of both that can Solo some endgame bosses perfectly fine though, you just have to put the right combination of effort into it.
However, if you truly love your knight, I think you should consider working on it. Heal pets are not too difficult to farm in a day or two. Willow Worker will be the cheapest and fastest to farm. In a raid with all the tank classes, top threat is usually Warrior and Knight, BMs and Monks have a slightly harder time catching up. I can see knights who achieve phenomenal results in end game, and you can read around to see how they excel.
Experience: Still a free to play Beastmaster since day 1, can solo all but i think 5 bosses, which are of course not meant to be soloed. Fun things to solo include WoE Guardian Leaders and Himmelmez's Dragon. Can also go into relatively dangerous areas all by yourself if you'd like. I use pretty garbage gear, but still perform just fine.
Edited by Arbalist, 08 January 2015 - 01:26 AM.