A 48v 17A controller will give you 816w on maximum. For this you would need a battery capable of delivering at least 20A. Note this 20amps not 20Ah, this is the capacity.
Here's an AKM100cst 250w as an example. Although rated at 36v, if you bought the 201rpm version and ran it at 48v it would give you the rpm you need.
Zen Cart! AKM-100CST Cassette 36V250W EBike Rear Driving Hub Motor 32/36 holes [AKM100CST EBike 36V250W motor] - ★★★★★ AKM-100CST Cassette 36V250W EBike Rear Driving Hub Motor Specification: 1. Voltage: 36V. 2. Brake type: V-brake, rim brake and disc brake are compatible 3. Connector: 9Pins...
www.topbikekit.com
Of course it depends on gear mounting, cassette or screw on.
Although , as a first time conversion, it might be wise to give Woosh a go. You'll get all the advice you need prior to buying.