A bit about my plan as context:

I want a medium sized box that can be locked where I can put some of my stuff in on the travel to conferences so I don't largely increase the room I take up in a train. On that box one should be able to sit at least somewhat comfortably with a retractable back rest (made from cheap wood and a few hinges) because back rests can be difficult to find and I need one when I spend much time outside.

I don't really want to prioritize speed because one of my main use cases is to have somewhere to sit during long queues (like at the badge pickup) because my legs cause a ton of pain when I stand for too long.

It would be cool if it were controllable with a gamepad but that's a part where I believe I already have all the skills necessary to figure out how to do it on my own.

So my main question is where can I get wheels and motors (one servo for control + one motor for propulsion) that are strong enough for this while still being affordable?