ia have a couple KJ, metal slides and like them. i have had a breakage nut the maker is resonsive and sold me spares. its Tkio Marui compatible also.
I got a TM G18C and is pure crap. I'm havin to replace everithing starting with a metal slide and outer barrel. i tried to put in some fiber optic sights but it doesen't tke parts from TM G17... Now it has broken the firin mechanism.. I think qhen finised replacing everything I'll get a decent tool but that will cost me mucho more than a real firearm.
only i can do some practice that's impossible with a real gun, like getting used to watch the from sight moving around.
There are good reources on youtube related to assembly/dissasembly and replacement parts.