Sorry. I started with the sensor and then forgot about the rest.
For a 250w motor,you'd be hard pressed to tell the difference front or rear if you were blind-folded. IMHO front drive is safer than rear; rear is quieter; rear gets more traction, but can wheely on very steep hills. If you have nice gears and changers, it has to be front-wheel drive, so rear is better on bikes without decent gears.
A replacement battery would be expensive, but they use 18650 cells which you can buy on Ebay, so I'd re-cell it and get more capacity. By the time the battery needs replacing, the cells will be up to 5aH each, which would give you a 20aH battery of a similar size and weight. Already there's 4aH ones. And I bet that you'll be able to buy standard bottle batteries with a much higher spec. The case is standard. A BMSbattery one will fit and only costs £150 delivered, but doesn't look as nice; however, you could easily swap the insides.
If you have pedal sensor operation, brake switches are essential because the motor runs on for about 2 seconds after you stop pedalling.
There are no readily available brake switches for Shimano hydraulic brakes. You have to make your own.