A question; Front wheel drive is fine for product made for it. Non ebike bicycle forks are not designed for the continuous load that a motor puts on them. Rear drive with the strength that the triangle affords are no problem. Have there been any failures of front forks using a retrofitted motor?
I think you be confused with terminology. Ah typically refers to the amount of power a battery can provide to the system. Controllers speak of Amps and Volts.
What you need to know is the voltage the controller can handle choose a battery with that voltage and then buy a battery with the largest Ah rating you can afford. Also when looking at batteries its very important to know whose cells the Mfg is using when building the battery. I would stick with Samsung, LG, Panasonic.
Based on your other question regarding your controller, you need a 48v battery. The Ah rating is one I'd spend as much as I can afford. Before that tho, does the new battery fit on the bike without any new wiring issues... I'd go on a diatribe about battery sizing and what kind of measurements they are using in China but i've already said enough