G&G maxline guns were fully factory upgraded guns meant for the older import regulations. They were made only for the canadian market. As of last year, guns don't need to be as upgraded to be legally importable. You can use the extra money however you like.
Version 3 and version 6 imo are the easiest to work on. V3 guns often just require a couple pins and 1-2 screws to remove the gearbox, motor height is fixed with an attached motor cage, where as the v2 you need to assemble the lower receiver and adjust motor height every time you want to test cycle the gun. v3s are mainly g36s and AKs, there are a few others out there that are v3... some mp5s I think and possibly some of the sigs.
A v6 is only held in by 2 screws but I think only the thompson and the p90 use the v6.
As for compatibility... most guns fit the TM format in parts... it's easier to answer what ISN'T TM compatible
ICS split gearbox guns
systema ptw (and all the new ptws clones coming out)
PTS masada (gearbox by ares)
A&K masada (a few proprietary parts)
TM v6 is v6 specific, but every other p90 is made to use v2/3 gearsets and bushings. Everything else is V6 format.
While most are TM compatible and even some guns from the listed manufacturers have SOME TM compatible parts, they all contain proprietary parts to some extent.
JG g36 has a slightly longer nozzle than other g36s, but otherwise tm compatible I believe you can find replacements if you look though. JG gen4 or CA g36 if you want one.
CYMA makes a solid AK, cheap, well built some are even full wood furniture.
If you want m4s, I would spend more to get a VFC over G&P.
p90s, just avoid the KS/Echo1 version and you'll do fine. The TM actually has the nicest body out of the ones I've held, but the CA gearbox is nicer for compatibility. There's the G&G and KingArms models out now as well which may or may not have proprietary gearbox shells.
It's this forum's policy for members not to divulge locations of vendors to people who have not been verified (by volunteer community staffers) that they are >18 years old. You can google up a storm to find them though. I believe almost all the vendors that advertise here actually have a physical storefront and showroom now. Shop around a lot there's some really bad/overpriced vendors around, especially since the prices dropped by 1/3-1/2 last year due to the regulations changing.
200 is the garbage gun price range, you'd find just about everything you need in the 300-500 range these days, though 500 is really pushing it. CYMA AKs are priced really cheaply and aggressively imo so they're a good choice if you like AKs.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.