Externals = OK. G&G are more about bang for buck.
Internals = depends on version. Gen2 and 3 are the best (not sure if there are Gen 3 UMGs yet)
Versatility = Good. There's plenty of rail room compared to other guns of the same size, and they're suitable for both CQB and woodland- it doesn't matter about size, it may be an SMG but G&G's hop units (the most important sub-assembly for increasing range) are pretty good, especially on 0.25g BBS and 0.28s.
Mags may be a bit of a pain to find. As for pouches, there are P90/UMP45 mag pouches around which can fit a standard straight UMP mag.