Just ignore all the folks here who recommend using matched uppers and lowers, and go for it. So what if some of those people are advising that because they learned it the hard way? Odds are you'll be the exception. It'll be fine, just do it already....
You're already ahead of the game by getting a great deal with your lower. Why waste that sensible good deal by following it with wasted time and bad decisions?
Get a matching M5 upper and enjoy building a rifle without having to cobble mismatched parts together.
Or don't, either way let us know how it works out. If it's REALLY bad, we'd like pictures.