First parts to buy.
 

So its time to upgrade I have a King Arms M4 series and I am just wondering what should I start with? Just looking for advice. Thanks

Internal or External?

Externally, do whatever the fuck you feel like doing. Magpul ALL the things if you want to. Put on a free float handguard, turn it into a SOPMOD if you want, etc. The possibilities are endless.

Internally, run it til something breaks. At most Guarder Black bucking and barrel upgrade.

Coincidentally, if you want a 20" one piece barrel for a DMR project or something check the classifieds, I'm going to post something up in the next hour or so.

The ka m4 is a pretty good base to start from. I have one and completely changed everything except the body and barrel. Otherwise it's a totally different rifle.

hopup rubber, PDI-W, firefly, or Rhop
Barrel (up to 430mm, anything beyond that is a waste)
380fps spring
use .28g BBs
Then get a gundoc to shim the gears and fix any compression leaks in the mechbox