Which Monkey spares should I buy from the off?

[VintageRacer]

I'm about to order a Mad Monkey for my son and wondered what spare parts are worth ordering at the same time?

I'm planning on him using the motor that comes with it for a while. Is the standard pinion up to the job or will it need replacing soon (the kit pinions on my old Optima were soft alloy and wore out quite quickly).

I've read that the diff screw isn't that great and should be replaced and I'm going to get ball races from the start.

Anything else I need to think about?

Cheers.