Honestly, I wouldn't bother with any spare parts for lightly used TSDZ2 because in my experience they don't seem to have any particular weakness. I have two of them, a 48v 750w and a 36v 500w version. They both get ridden hard because I live on a mountain and they get a workout assisting me on every ride. The 36v I've had apart twice, once to make it coaster brake compatible (which involves changing the main gear on the right side of the casing and the reduction gear which is removed from the left side of the casing - after removing the motor and the blue plastic gear to access the reduction gear) and once to return it back to the stock non-coaster brake version. I'm not sure how many miles I have on the 36v motor but probably close to 1,000 more or less. The blue plastic gear still looks like new and nothing else had any sign of wear. That said, I did buy a spare blue gear to have on hand a while ago for $8 including shipping (as well as the straight cut reduction gear and solid main gear for the coaster brake conversion). I haven't read that much fails on these motors, blue gear and the main gear's sprag clutch apparently uncommonly. Buying all of the internal parts including the motor, gears, axle with torque sensor, controller and bearings would probably be close in cost to just buying a complete new motor for $400 (depending on seller) plus you would get a new display, wiring, speed sensor, crank arms and motor casing by buying a complete new motor. As well as the convenience of just throwing the new motor on the bike and waiting for a convenient time to work on the problem one.
In my opinion these TSDZ2 are an amazing value compared to other ebike options. Perform as well as premium mid drives (Yamaha, at least) and so quick and easy to install or buy parts and service if ever needed, which seems unlikely.
