Welcome! I too only do around 5,000 miles per year. In 2007 I got my 2004 Mini Cooper from my neighbours who had it from new and took good care of it. It is good fun to drive but not cheap if you rely on the garage when it goes wrong. (but then I'm a mechanical moron and I have a fairly low paid job).
I don't want to dampen your enthusiasm but do be sure that you love the car enough so you will put up with the niggles. I am a biker at heart and for me the car is really for practicality and if I want fun I'll go out on my bike. So, I don't resent spending money on my beloved old Ducati half as much!
I would search through the tech forum here to find likely issues - hopefully a newer one will be a bit better for reliability but I too would advise setting aside a rainy day fund and/or a warranty!
In addition to the usual servicing costs I've had an unexpected bill of Ł300 or so in just about every year of ownership: brake discs twice (they didn't wear out just rusted - a design feature apparently - although I think that's been dealt with on newer models), suspension, alternator, various leaks....
This forum has been a great help and led me to Mad4Mini in Leeds who have provided good value service. And I even managed to save time and money solving a couple of simple issues myself using the guidance on here: seat jamming and steering pump breather blocked.
I'd always had VW's before so I'd been spoiled for reliability. I like my "Mavis" and she's fun to drive but I don't think I'd get another Mini.
If you live anywhere hilly and cold I'd really recommend winter tyres too - they have transformed it in the snow and ice but I have a garage to store them in which helps!
Happy shopping :-)