Nooooooo that's a terrible idea lol
Buy a stock KA M4 then add on to it. Building your own gun will cost way more and be a huge pain.
G&P is also high quality for externals, but much more than a KA
Also good on you for having a realistic price range
Manufacturers all have different mentalities on how to build a gun. Like everyone makes a ~$15000 compact car, but some are total lemons lol