1. (Personal opinion) Stick with the M4...I'm the same height and find longer rifles a chore to get through tight spots. Longer gun might have some advantages but they're pretty much negated by just rushing up 10yrds and getting into the fight.
2. A nice 6.04mm (inner diameter) tightbore is a great bang for buck purchase. Buy a name brand quality barrel...don't cheap out there. Get a good hopup rubber at the same time.
3. For more consistent accuracy (and an increase effective
range) shoot BBs 0.25 grams
or heavier. 0.28g and 0.30g BBs are very nice.
4. The price of the BBs will be the least of your worries with airsoft $$$.....