Yes you can swap uppers and lowers around, except for some Colt models. Which is the best upper is kind of like asking "What is the best car"? Depends what you are going to use it for.

If you can find one in stock a Stag is a good bargain. See if Aimsurplus has them in stock
Personally I have an LMT on my m-4.
RRA may be cheaper, but I think their m-4 barrel is actually thick under the handguards where you can not see it, that makes it heavier, but also cheaper as there is less machining work.
If you are on a real tight budget then J&T probably has the cheapest quality m-4 type uppers.
Look around for a used one on the Equipment Exchange here.

Just shoot the whole thing with bowflage and no one will know what the original color was.

You could also refinish the upper in green to match the lower. If budget is an issue I would not worry about it. The AR I use the most as a light grey lower and a dark black upper, doesn't matter to me, you can only see the sights when you're shooting it.