KWA m4's "out of da box" are some of the most consistent guns iv seen with no upgrades needed. They are high end in price but need little to no work to outperform most other "stock" weapon systems. They can be found right around 300 at the right place depending on exact model. Classic Army has plenty of m4's all over for a good price, however all 3 CA M4's I have bought needed some love n upgrades to be up to par with KWA's Stock low end guns. If you enjoy a project gun that over time will cost alot of money become your fav airsoft gun I'd say CA is a great place to start. If you don't want to put any money into it over time and want it to still be reliable "out of da box" KWA is ur best bet.
Iv learned the more money you spend on 1 gun the more you will use it regardles of brand or upgrades. Just look at how much exp people have buying and upgrading guns befor you take there advice. Alot of people have 1 bad expreince with a brand and will bad mouth it forever. Iv worked on and paid to build multible M4's and they all do great regardles of brand or branded parts. It's about where and how you spend ur money. Buy a nice M4, and WHEN (not if) it breaks upgrade the broken parts, not just replace them. Like I said the more money you spend on 1 gun the more you will use it.
P.S: don't get a GandG plastic model gun. Aint a hater on GandG, there sig's are insane "stock" guns. But there low end guns feel like crap and are not worth dumping money into when you could break it by simply droping it. I can spot a crapy GandG from 100 feet away by sound and looks when all other guns blend and perform relatively up to par wit one another.