I've had quite a few mini's of varying ages and currently run 2 late model MPi's. Personally I'd leave the 12" wheels on. !3" wheel cars definately don't drive as well as 12" wheel Mini's. I run both mine with 165/60 x 12 tyres (which don't needs any mods or spacers) which fill the arches nicely.
Look carefully for rust but some suffer worse than others. One of mine is immaculae with no rust anywhere and has never seen the inside of a spray booth (although it is low mileage, garaged from new and SORN'ed every winter)
My other MPi has done 112k miles, gets driven every day regardless of weather but has had work on the scuttle, sills, floors and A panels. The gearbox on this one has just started jumping out of 2nd occasionally but it's the orignial box and has done a lot of miles.
I'd have to say that MPi's are probably the easiest to live with as a daily driver and the 2.9 diff means thy keep up well with modern traffic and are good on fuel too.
