Budget is the factor on dmr . Like most airsoft stuff
Hpa is probably easiest to use for dmr
Electric . Nice stronng built gearbox with good high torque motor and mosfet . Barrel about 400 inner and rotary with maple leaf and flat nub a good start . Gun wise sr25 is about easiest to do . Plenty of them about form cyma to classic army . Or the 417 series from e&c is nice . Mdr is good out he box too and nice bullpup 308 rifle . And it's already upgraded from factory