Ok so there are at least 2 brands of tires, are the mini wheels cheaper than Mitsubishi steel wheels? Are you getting them used from a scrap yard, or buying them new from the dealer(s)?
a set of used 15in mini cooper wheels usually go for about $200 on craigslist if you can find them. you can also try looking on mini cooper forums. scrap yards will have them, but they seem to want $400 per set.
you will need a 10mm spacer up front and longer wheel studs also for the mini cooper wheels to fit.
I think the michelin x-ice xi3 are an interesting winter LRR tire - available in 185/55R15 front 185/60R15 rear.