Advice on Centro 4.1 for my son

[oldschooldriver]

I last raced properly in the early 90's and I've recently got my son a 2nd hand Centro 4.1. I'd like to go with this while he learns to drive. It's got a geared diff (I think) and he'll be driving it indoors mainly on carpet. I want him to learn car control, so I have turned the end point down to get him used to it, and will gradually turn it up as he gets better.

The Centro 4.1 has got a Pace 45r esc and a b/less 17.5 and a couple of packs of shortys. I've just had to invest in a new servo as the cheap 2nd hand servo didn't last too long, so i have invested in a Savox servo with metal gears.

Anyways, I want to be careful in what I spend on a 2nd hand Centro 4.1 (If I had the budget i'd get him a new B5m lite). Any advice on what to upgrade or what is a waste of money? Upgrade to ball diff? Big bore shocks?

I have just invested in some yellow minispike2 rears and some cut staggered low prof yellow fronts. I'm not sure if I should bother upgrading to Hex adapters front and rear and new front axles and then getting new wheels?

Any advice is really appreciated, as I don't have a lot of money to make expensive mistakes!