You can take a cheapo gun and have it perform really well. In any one instance, it might take a lot of work/parts/etc...it might not take much. In general though...the end result is usually "nicer" if you start with a mid-high range base.
The difference will be in the general fit/finish of things, (fewer) compatibility (issues) with other parts, etc...
With the "lower end" stuff there's some models that are really quite good...but others that are just crappy. There's more consistently decent models with better manufacturers.
G&P, King Arms, VFC....they're all reasonably priced.
AK's would be a great one to start with...they're very solid...a straightforward/simple assembly. Compared to an M4. you'll spend less time with fiddling getting the mechbox in/out of the body and more time fiddling with the mechbox where you can tune things however you want.
Don't start with odd-ball guns like a FAL/Thompson/etc...