Depending on your budget it can vary. But from what I've heard from a lot of people is that the GHK platform is a solid gun to build up from. Keep in mind this is the high-end budget wise for both the gun and mags. Get age verified and you'll find used GHK m4 and mags on the classifieds every now and then.
If budget is no issue, I would recommend building on the WA platform
