The late R53s, of which I bought one are mechanically very sound, and feel very good.
If budget is an issue, go for an R56, 'cause the R53 is not for the scrimper!
I had an R56 Cooper for a week, and although the car was easier to live with, and very refined (yadda, yadda etc...) it was just a little dull, too easy and not much of a buzz (still a very, very good car though) - do what is right for you at this point in time!
Good luck,
Jay
R53
MCS PW/B