If you’ve ever worked on a Mini you’ll know why it’s not further up the placings; there are some jobs, even replacing a fan belt, for which the Mini’s compactness makes for skinned knuckles and furious swearing.
But parts availability and pricing wins through, along with the simple construction of the bodyshell. The design is generally extremely clever though rust-prone of course. The separate subframes aren’t easy to replace but a new subframe is better than having to weld up a rotten chassis rail.
And of course the A-series engine is an extremely well-known quantity, even if the in-unit transmission can be a bit fiddly. As long as you opt for the rubber cone suspension rather than Hydrolastic, there’s very little that will phase you mechanically.
Best of all, the Mini is more enjoyable to drive than almost any other classic.