I ran a business building and supplying custom e-bikes (mainly to the British Police) for several years before I checked out.
Firstly, dedicated e-bikes are often junk. You are fair better off sourcing a decent mountain bike (something with hydraulic disc brakes) and converting it. The aforementioned Bafang mid-drive motors are very good. You get the benefit of the existing rear gears on the bike. With hub motors you lose them. I'd go for a 48V motor. The motor controller offers pedal assist and throttle options (or both).
Regarding batteries, I'd opt for A123 LiPos, they offer between 3000 and 5000 charge cycles. You will be lucky to get 300-500 charge cycles out of batteries supplied with most cheaper e-bikes - they want you to buy replacement batteries.
You should be able to buy a bike and convert it for about £1,200. Something like this:
A comparable ready to go e-bike will set you back thousands. If you want to see some crazy ones, check out Stealth.