I suggest you buy one. Live with it and learn from it. Then build what you want. You can get one for $800 or less.
I don't know about your state laws.
I have an Olympic Arms AR-15. It's always worked just fine, and I am relatively sure you can get one within your budget.
As to your questions:
"DI vs Piston Driven?" At the price you are willing to pay, it's going to be DI, so it isn't an issue.
"Mil-spec vs commercial stock?" I wouldn't worry about the difference. You can always replace it later.
"Is forward assist important/needed? Ever utilized in realistic scenario???" I never needed mine. It doesn't hurt anything though.
"Dust Cover? (Seems like less of a concern)." You want the dust cover. In fact, you should get in the habit of closing it when you are moving around in the field.
I'd find out what the rifling is. Military is 1:7. Most commercial rifles seem to have 1:8 and !:9. It will affect the muzzle velocity and the best weight for your bullets. Mine is 1:9.
Don't over think this. Pick one, have fun and get experienced. If you want another, it won't be hard to sell the one you buy.