Was thinking of buying a rock river arms M4 style flattop upper to attach to my BM shorty A2. Will the fit and finish be okay. The BM complete upper is a little pricey. What's your thoughts?

The reason that the BM is a little more is that they are Mil-Spec. Chrome lined bore and chamber, where the RRA is not (as far as I know). But all I've heard is good things about RRA.
But I've been thinking about getting one of the LE carbine uppers with the mid length handguards (the new LE i peice upper, not the one with the detachable carry handle).
But all i've heard is good things about RRA.

In the RRA Forum people have mentioned putting a RRA upper on a BM lower with no problems. I have the flat top upper you mention, but on a RRA match lower.
I ordered mine with the detachable front sight and Weaver style gas block. Great piece! It is true that they do not have chrome lined barrels, but that will make it more accurate.
You will have to decide which is more important, accuracy or a little easier cleaning.