That price will be very difficult to meet. For that money it'll probably have to be an AliExpress special. Get a front hub. You probably want a torque arm but I don't think you'll find any in that price range. You can get away without one if you run the motor at low power, like 250W. Unfortunately I don't think you can get a non-hazardous battery anywhere close to that price range. Perhaps look for LiFePO4 (LFP) battery since the chemistry is inherently safer and it's cheaper to make. It's heavier than standard Li-ion.
Otherwise I'd get:
- Bafang G311
- Compatible Bafang controller with torque assist capability. The lowest wattage will do
- Bafang display
- Bafang torque sensing bottom bracket
- Good quality but smaller battery. E.g. 300-400Whr.
With all that said, steep gradients can kill geared hubs so a direct-drive hub or a mid drive like BBS02/HD might be better suited. Most DD hubs are more powerful so you'd either have to use torque arm/a or run it at low wattage. In general aluminum frames can't handle a lot of torque at their dropouts without spreading or cracking. The rear dropouts can handle 500-700W when the nuts are correctly torqued. You can read about that on ebikes.ca.