Well first you've got to provide more information, we have no idea what you have. For instance, do you have a handguard end cap inside the gas block? The MOE handguards need it. As far as a flip up on a gas block, they do make them but what height rail do you have? Too many unknown variables here.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek