Both of my kits were from YescomUSA, one from their site and one from their ebay page. I used this model.
26in Rear Electric Bicycle Motor Conversion Kit 48v 1000w – yescomusa
Regen is something I wanted as well, but the general consensus is it doesn't help much for ebikes and I could only find it in more expensive kits. The kit I used supports and comes with the parts to make it pedal assist (throttleless) using a sensor attached to the crank. I haven't tried it out, so I don't know how well it works. I believe the throttle will override the assist if you don't feel like pedaling.
Acceleration won't be an issue if you want enough power to do 30MPH. Mine will keep with traffic till about 20MPH on its own and I can leave cars in the dust if I'm pedaling. 1000w plus whatever I add will get a bike up and moving pretty quick.