First time builder. please have a look and share your knowledge

[Yatcatcher]

I know it's a pain, but the easiest way to build a chassis is to make it last after you have all the parts you are going to mount inside....

As to cheap (ish) options, 4QD aren't used much nowadays as the control isn't the best but give good reliable and cheap controllers with an optional RC control - work with low powered motors well. Also have current limiting, good with cheap motors!

For drive train, talk nicely to someone down your local Kart track for light weight wheels and tyres, drive cogs, chains, hubs, axles and bearings. All well able to work reliably with less powerful motors.

Motors, difficult decision. Tempting to go for American style Mag motors or Amp motors. Best budget motor is still perhaps the venerable Bosch 750 gpa motor. Run at 36 volts it has good power. The power is quoted in watts on a motor (the Bosch is rated at 750watts at 24 volts which is approx. 1HP). I think it's a good idea to buy a slightly better motor and not drive it so hard if you're on a budget so it will last.

Batteries - the advantages of Lipo seem to make them inevitable, just the savings in weight, the extra current and the small size. But they're not cheap. Gel lead acid are cheap but add 10 kg or so to your robot. They do limit the current too, but with cheap motors that's not a bad thing!

Spinners? The force you put into the opposition also goes into your robot through the bearing. Big motors, big controllers, big batteries, expensive materials. I'm sure it can be done cheaper but the successful spinners are big investment (time and / or money / machining) machines. Also if you're doing this with your kids, spinners are capable of huge harm.

Wiring? A very basic diagram attached.