I gathered a small mobility scooter chassis and motor some time back but lacked a sensible battery to drive it.
With the explosion of popularity in eBikes, there's a lot of controllers and battery packs making their way onto the secondhand market and one of the people at EMFCamp 2022 was selling a stack of them for project use.
This is a 'SilverFish' battery pack, a quite common but not entirely standardised design that packages the batteries and BMS into a nice solid removeable pack.
At a nominal 36V it will overpower the 24V motor I have but not excessively so if it's under light load. I've a charger on the way for it and will need to investigate a suitable speed controller.
I'm not thinking of building anything at all autonomous, that's a serious exercise and in something large frankly dangerous, so it'll be traditional radio control gear in use. I am however thinking of making it have FPV video and perhaps some automated collision avoidance using LIDAR so it can be driven without having to follow it.
If the chassis I have doesn't doesn't work out, I'll probably switch to one based around four hoverboard motors doing tank steering as that's a common hacky rover design. Making it usable away from paths will mean modifying the hoverboard hub motors so the wheels are a larger diameter, but again that's a well covered thing.