With 250W motor, you can get up to 20mph derestricted, 350W up to 22mph, 500W up to 24mph. You can run 36V battery up to 500W.
for fun bikes, it should be the 48V BPM CST motor. Total motor + battery about 8-9kgs.
But if you want 30-40mph, even on flat roads, you need 1,000 to 2,000W DD motor...