She doesn't know? You are a very brave man indeed.
My advice is to go at it emotionally, not logically, because that's what her reaction will likely be. I mean, how would you feel if she did it to you? (not a MINI, but buying new furniture or something?) You'd better romance her off of her feet - buy her flowers, take her out to dinner, whatever. Then, once the time is right, you gently break the news explaining how important buying the MINI is to you. Good luck!
How about giving her the impression that it is something far more serious - you have been having an affair, have 3 months to live etc. That way when you tell her about the MINI she will be so relieved that she won't have time to get angry.
Either that or begging pathetically - it usually works for me
After having a test drive in a Cooper I explained to my girlfriend that I was going to buy a MINI Cooper next year. she immediatley had a fit about how expensive, small and unpractical it would be. However since going to the showroom where she instantly fell in love with it she now wastes no time telling all our friends and family members WE are getting a new MINI, and she hasn't even driven one yet!
The problem I have is that I am a real Mini nut. I have owned 5 before getting married and haven't even driven a new BMW one. I ordered it just because it looked so
damned sexy and haunted me for weeks after stumbling across 100 of them parked up in a Manchester multi storey car park prior to the launch date. I never even thought about what my wife would say! What really pisses me off (is that allowed?) is the fact that it has gone into production without the 17" rims I dream of (all because of a pushy dealership!!!).
Tell her you're having an affair with her (sister, neighbor, brother..whatever will upset her the most). After the crying / hitting / shooting begins, tell her it's a joke...all you did was buy a MINI.