Palmetto state armory often has great prices and great products. Through them you'll be able to get a nice rifle for around $700 (+/- depending on what sights you want) ready to shoot. This is with milspec built and tested barrels and bolt carrier groups, if those things matter to you.
You could even buy an upper from them and a lower locally. That might get you the best price.
DI vs piston is a heated debate in the AR community. Luckily (
) for you, they're way out of your price range, so you don't even have to worry about it.
You honestly don't need a forward assist. Both of my rifles have them though.