ok, take two after my browser crashed...lol.
automags are very customizable; you can set them up (bodies, rails, grip frames) almost however you want. some parts are specific to other parts (ie, a frame or body that only fits certain rails). it should also be noted that the automag rt classic is its own thing; the body rail and valve only work with eachother.
that said, there are really only two kinds of valves, classic and reactive. they differ in how they work: classic valves regulate the incoming air before it hits the on/off valve, while reactive valves regulate the air after it hits the on/off valve. this allows you to "sweet-spot" the trigger when set up right with a reactive valve, and allows for very high rates of fire without electronics. indeed, "rt" stands for reactive trigger. if you've ever used a tippmann with a response trigger, the effect is similar. it resets the trigger faster than you can, with greater force than it takes to fire the marker. so with the right amount of force, you can get it to bounce. some quick videos from my youtube channel showing off some reactive valves. note that it's easier doing it with two hands lol! custom tac-1, retro valve and a micromag retro valve.
classic valves include the 68 automag air, 68 automag classic, minimag and 68 micromag, as well as some specialized / rebranded valves (smart parts, hypermag, centerfire minimag). they all work the same, they just have different names. they all all made of stainless steel.
reactive valves include the rt classic, rt pro, retro, micromag retro, emag and x-valve. these all work the same. they all have aluminum regulator bodies to save weight. the x-valve has an aluminum valve body as well to further reduce weight.
there are also two kinds of bolts, level 7 and level 10. a level 10 bolt is a mechanical anti-chop bolt; in the event of a mis-fed paintball, the bolt will hit it and reset instead of chopping the ball. however, the bolt can still hit hard enough to crack the shell, which would result in a barrel break. the level 10 bolt takes a little bit more time to set up and tune correctly, but it's not hard.
as for maintenance, other than oil and chaning an o-ring every now and then, automags are bomb-proof. super-easy to work on and troubleshoot for the most part, and very durable...some are 20+ years old!
as for what to look for, that is personal preference. do you want a solid mech? do you want something higher-end and lighter? do you want semi-auto? do you want a reactive setup? do you want autococker threaded barrels? do you want vertical feed? you can get a basic classic automag for less than $100, or you can get crazy with a custom automag with sick anodizing for $1,000!
also, if you don't want to watch an hour+ long video, you can just watch this condensed troubleshooting video.