Re: AC or DC Configuration
Tough question. Strange vehicle. There probably is not a simple answer. Efficiency seems to mean different things to different people. To you is it the classical power out over power in? An instantaneous value. Or is it a drive cycle type of thing? Total energy consumed for a specified distance.
AC or DC? A lot depends on the particular equipment in question. If money (and time) was no object, AC could likely prove the more efficient. But you always have constraints. So what you can actually obtain may not be clear as to the efficiency winner.
I don't often recommend PM motors. But you have a small vehicle. You might consider a brushed DC PM motor with a minimal control. Like a starting resistor and run direct to the battery, or PV, without a controller in the circuit.
And, 400-1000 RPM for the motor is probably not an efficient choice for motor speed. Yeah, you can get motors designed for that speed in the power you need. But they are likely to be several times the mass of a higher speed motor and reducer combination. So, does the mass of the drive system enter into you efficiency figure? Not if you use the Power out/Power in. But it would for energy per mile if you have accelerations or hills to contend with.
No easy answers from me, but maybe some things to think about.