V3 is supposedly easier to work on but I've only worked on V2's all my life. I don't do work for others only on my own stuff so yeah it's just work on V2's for me which I find fairly easy if you can easily grasp mechanical design concepts.
Also, if you want to learn how to work on internals it might be more cost effective to just buy a drop in gearbox (or gearbox shell, but the danger of piecemealing the shell and all of the other internals is that you're spending way more money than if you bought a drop in gearbox and a few key upgrade parts) and just work that portion irrespective of the rest of the externals. Obviously you won't be able to do hop up/bucking/barrel since that mates up to the gearbox from the front end but for the rest of the internals; spring, ARL, compuression parts, gears, tappet plate, etc. it'll be perfect.
My vote goes for either the VFC or Guarder shells and just build up from there if you want to build your own, otherwise buy a complete gearbox and put in a few key upgrades but then again there's not much you need to do in a VFC gearbox since as far as I can tell they're solid gearboxes;
and here's a review of the retailer in question; http://www.airsoftcanada.com/showpos...6&postcount=25
Also it should be pretty obvious by now that I'm well acquainted with the staff at that specific retailer (being a player for this amount of time it's pretty hard not to know about the different retailers). I give them my seal of approval as well.