So propane is less money but they last a lot longer, ed is the one to ask about this, and all gas gun related. Try go outdoors, they have the best prices apparently. WE hk416's are compatible with ra tech, and I am looking at them currently but haven't made my mind up.
I have heard the TM MP7 is better than the kwa but yanks hate to admit it (kwa seems to be more american the tm in that most people over there have kwa pistols etc) but apparently it has a nicer feel, better externals and internals are very good as well.
GHK do an AK range, never heard about them but one would assume they are good because they are GHK. However, I have no idea of this, and this is a one off so don't get mad, arnies airsoft has a thread about them in the reviews section, something like 25 pages so you might want to have a little look at it.
Apparently the WE mags (from eds video), he said that the current gen (gen 2) mags are a much easier repair, and are better at holding gas. I would send him a PM, he obvs is very good with that stuff.
Also, a lot of lubing, greasing and maintenance goes on, however I look at it as part of the package and not a chore, depends how you see it really.