Spin,
They are still readily available, and reasonable prices as well. The main advantage is that because it was such a popular car, there was never and still aren't any problems getting new bits for these motors. In fact, I think you could build a brand new one by ordering everything from the catalogue.
I used to have a accountant aquaintance, who was Alex Issigonis's right hand man when he was designing the very first minis, and he said that he was so hot blooded (Greek extraction I think) he would not let anyone deviate from his original drawings. It wasn't until after the initial ones were built and sold that they found major bodywork problems, with the outside seams being made to fit the wrong way up, and letting in copious amounts of water into the passenger compartment. So if ever you went for one, stick to a mark after 1965, by then they had most of the major problems sorted. I can't quite remember when I saw my first one, but it was either 1959 or '60, it was a 'tallyman' getting out of it, and he must have weighed at least 15 stone (210 lbs). Stew and myself had a little bit of a discussion on this dating of the first one when he came to visit, but now have found out that it was released late '59
I personally don't think the latest BMW version captures the spirit of the earlier versions. They are just too large.
John