IMHO disc brakes are necessary if you're going to use a 350w or 500w motor and its associated heavy battery. Rim brakes are OK on light-weight electric bikes up to 15mph.
Cable disc brakes often get a bad reputation because they're not properly adjusted. I've never seen a set yet that came from the shop OK. They require a lot more adjustment than hydraulic ones, but a lot less than rim brakes. Another important point is rim wear. If you look on Endless-sphere. you'll see several instances of rims worn so far through that they can't hold the pressure from the tyre, so they pop off with catastrophic results. This sort of wear takes a long time on a normal bike, but it's different on a high speed/power electric one.
The bloke from BMSB was saying that you can't use the kit's e-brake levers with your bike because they replace your existing ones leaving you without gear-changers. You therefore need in-line brake sensors, like their Hidden Wire Brake Sensors as an additional item instead. There's no reason that you can't choose disc or rim brakes with that motor.
A 250w motor is OK on the front, but if you want a high torque motor like the Bafang BPM, it would be better to fit a rear one.
You mentioned a 10 or 12aH battery. There's not many that have enough power for a 500w motor.