You will be missing out, I had a r53 cooper s before my ed30 and now have a r56 jcw one after I sold my cayman, and they are great fun to drive. Take it out on a decent country road and I guarantee you will find it alot more fun and involving than your golf, its not as fast in a straight line but they are very neutral and well set-up out of the box even on run-flats.
40,000 mileage is pretty good for that age so I would just look out for the normal stuff like you would with any other car - has it been modified, gearbox ok, brake pads and discs etc and does the supercharger sound alright.
You might find it quite a crashy ride compared to the soft set-up of the golf but the sharper handling and better grip makes up for it, and changing the tyres to non runflats will improve the ride considerably.
If she gets it, and you can cope with driving a girly car, then take it out for a hoon and report back.
as for mini forums - totalmini and minitorque - seem the better ones.