the ka m4 is probably the most economical starter m4.. it's quite basic though with no rail system only the rail under the carry handle. If you know exactly what rail system you want, you may find it in the g&p or vfc lineup without having to rip apart the externals of a brand new gun.
Like all out of the box airsoft, there's a chance of things failing, mainly the piston. Beyond that the cutoff lever on the KA is a pretty quick wearing part. I've changed multiple levers on guns within a year of purchase for people. It's a cheap part though so that's not much concern.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.