The revo style is one of the only monster trucks that you can hydro halfway easily because of its low center of gravity (it's essentially a large buggy with monster truck tires on it). Be very careful buying used, it's much like buying a used auto i.e. the more knowledge you have the better chance you have of catching something fishy. $500 for an old revo is prolly not a great deal but if it's packed with upgrades and was rarely used then it could be a decent deal. As you suspected the slash isn't the best at anything aside from it being really forgiving as it's tough aside from a few cheap and easy upgrades and is also very user friendly while still being stupid fast with the right set up (my 2wd does 60mph easy). Another thing about the slash is that parts are everywhere and plentiful. The slash is a great middle ground when compared to other niche rc trucks. The biggest reason most people put the slash down is because it's not good for racing for a few reasons but if you're not gunna compete then you will enjoy it. The reason i recommend the 4wd over the 2wd is because the 4wd will keep you interested longer as it is more capable, especially in high speed turns. The 2wd on the other hand will teach you to be a better driver as its control factor will be lower but like you said. Once you master it you will be bored of it leaving you wanting something more. The 2wd WILL hydro just fine, i have done it many times and it so much fun but taxing on the car and you need a particular kind of spot to do it. It's really all about your personal goals, for me i wanted to just bash but then i hit the track ONE TIME and was addicted lol. As for other trucks to look at the losi baja ray (this one has an interesting solid axle making more squirrely but very entertaining) and the team associated sc10(2wd) or ProSc(4x4) would be good rtr candidates. Just be warned that the supplied chargers and batteries that come with either will not be good for much of anything so factor a good lipo charger and a couple good lipo batteries into your budget.
Alex