I dunno much about KA guns, but their AK parts I can personally vouch for as good.
The CM.028B would be a gamble - it would be powerful and last ages whatever, but it might not be accurate OOTB - you might have to fiddle with the hop or something - you should also be aware that because the materials are cheap, you don't want to be taking it apart a lot or the screw holes will get damaged and the little latch bits that hold the battery tray on (which are the definition of bad design BTW but nothing a bit of LX tape wont fix) get snapped.
- off the top of my head I'd say that, if you don't mind it being a tad noisy, a spring downgrade is all it actually needs (and tg will do that for you). Trying to make an .028 quiet is a bit of a mug's game though, i've found to my cost - the plastic receiver is always going to let a lot of GB noise through so basically you have to go with a silent head set and/or sorbo pads (which seem to be a real ballache to make stay in position), a good shim job, and regreasing and even then it wont be quiet, just only as noisy as an an average decent AEG!
- the gears don't look like much, but so long as you detension the main spring (by reaching inside the GB with a hook made from a bent paperclip, up through where the motor was attached, to pull the anti-reversal latch a couple of times to let it spin backwards) before opening the GB shell every time you do it, they'll do fine even at silly RsOF.
- if you should feel tempted to replace the air seal nozzle, be aware that you need the short type replacement and if an ad doesn't say long or short, it will be too long. 19.7mm is what you want. Coincidentally enough I've just found
a source for ali ones with an O-ring, but I haven't actually tried them yet so i can't say y/n.