For your Young Guns class, the cheaper the better as you ARE and WILL be bending up wheels and cutting tires. I'd suggest staying with the stock diameter wheel size , so that your power train is easier to tune. Obtain 3-4 FRONT wheels and run stockers on the back.
You are going to have a different package on the Mini when you step into them, so reusing your YG wheel/tire combo will not be advantageous, but instead may not even work with your next car.
As per oversize lug nuts for metrics, I don't know of any source. Ask around the local racers in your YG class to source them.
Whatever you do, do NOT overthink your car this first year. Just make it SAFE and RELIABLE, learn to drive this hot rod, and HAVE FUN !
A note; Your driver has been used to rear drive and is moving into a front drive. He'll be having to RELEARN everything he's already used as it's a 360degree difference.
