A g&g cm16 is a good gun, although the externals are plastic, it's really good quality plastic and very hard wearing.
The internals are the same as their top tech range and are very reliable. The combat machine range is possibly the best budget range out there and they cost around £130-150.
Then I would buy 2 or 3 7.4v lipo batteries, a good smart charger and some decent magpul spring fed magazines plus some protective gear and possibly that will leave you a few quid left over for some extra goodies or perhaps a nice pistol.
Not a fan of m4's but it is a very versatile platform and highly customisable.