A 2006 convertible would still be the original R52/3. The replacement came out in 2009.
I had three hatchbacks (One, Cooper and Cooper S). Bear in mind that they first came out in 2001, they’re feeling pretty old these days and are pretty darn small. However, I think the original BMW MINI is by the prettiest of the three generations by some distance.
They weren’t without issues but by the end of the production run, they were pretty good. The engines are pretty simple so any niggle shouldn’t be too painful (the replacement cars suffered from timing chain issues – the dreaded death rattle). Steering columns do fail and the electrics can be problematic. The gearboxes on the very early cars were notorious but at the age you’re looking at you should have latter gearbox which was more reliable.
Of the three I had, the face lift One was bay far the most reliable and it had done loads of miles. The only issue I had was the clock gaining time. My Cooper S was a bit of a lemon but was dealt with by warranty. The face lift cars gained little updates like low washer fluid warning and trip computer, revised headlights etc which the convertibles should all have.
The original cars have much heavier clutch, gearbox action and steering than the second generation versions. The steering is noisy but it really is a case of they all do that.
I really loved mine (aside from the One but that replaced the S when I changed jobs and needed something more fuel efficient). Having had the One, I don’t see the point in buying it – running costs are virtually the same as the Cooper (same engine, detuned with a more restrictive exhaust to further reduce power) but noticeably slower. The Cooper is the sweet spot for running costs and performance IME plus you can get the Chilli pack which brings nicer seats etc.
They shouldn’t be too bad to run these days because there are loads of second hand spares available through the likes of Mini Matt that can take the sting of costs plus lots of very good specialists.