I am not sure where you read that shocks are a necessary upgrade. If you can still find the rc-mini web from Australia (they are shutting it down soon if not already), you will see there extensive testing and reviewing by racers at the top of their game who will tell you nothing is needed to upgrade a mini. Shocks the least of everything.

But for my personal view, if there was anything I had to change it would be the shocks, and since Tamiya's only strong point in upgrade stuff is that they have one of the best shocks on the market today (compared to other OEM top manufacturers), I would say you made a bad choice and just missed that opportunity.

Also, from my experience, most third party "upgrades" for Tamiya cars in general are inferior (if not downright dubious) quality. There are differences of course, but some of the best stuff still comes from Japanese third party companies. Very little of it is worth the money, though.

Oh, and I did spend a lot of money on shiny Tamiya (mini) stuff, and yes, it does look nice.

Tires are glued to the rims, so if you don't want to throw away every tire after testing it once and the pain of removing tire glue form rims, I would say you need a set of rims for each tire.

Tamiya cars are renown for what is endearingly called "tamiya slop" and they work just fine that way, especially minis. If you come from a TC background (I assume not), you might find it unsettling, but you just have to go with it. Or spend hours on end trying to get rid of it and shimming everything and trying to find the perfect slop free balljoints and cups and play free bearings. You will spend a fortune in the process too.

As for ground clearance, minis require the lowest possible, otherwise they like to tip over very often just to annoy the heck out of you. This doesn't affect their performace though, because it seems they go on the roof just as fast as on their wheels.