Well for what it's worth, I built a really nice M4-gery for under $900.00....It's not "Supermoto Approved", but enough folks around here have had their hands on it to extol it's virtue...I think it will compete with any off the shelf rifle - in form, fit, function, reliability and especially accuracy - this thing is a laser beam.
Look into a Sabre Defence barrel - I have am M4 1:7 and it's really nice
BCM also puts out a nice barrel and it's where I bought my BCM M-16 BCG - a must have for an M4.
With lowers and uppers being easy to find and fairly consistent in quality, you can add a decent LPK and slide in around $87.85 for the upper receiver, (BCM) and complete, (you'll need to pin the stock) lower BCM's going for $350, you're on your way. You can also truly do it yourself and buy a stripped lower from S&W, Remsport, etc, add LPK and get to it...