Galaxy is the cheapest one but good value for the money. Only they make that, it's not the same as JG. JG has metal body too. The current cymas are awesome, but heavy as a brick - of course, they are metal. TM is very good but expensive and full plastic. IMO they don't worth that price.
The old Cyma 027 are plastic, pretty good, light and well worth the price, but those are mp5, not mp5k.
I don't know if G&G, ICS or other brands make mp5k or not, I never had those.
So if you need light weight, and it's for CQB or secondary, get a galaxy one IMO. Clean, reshim, regrease it, and put a new hopup rubber in. Put a deans connector on it and use 7.4 Lipo. Don't change anything else. The motor is weak, but fine with the factory spring, the bushings are plastic but they are fine, metal bushings are not needed here.
For a primary gun I would not use anything without a stock.