Hope it's not this one: https://www.carandcl...om/car/C1810884 ?
Rust advisories start as early as 2009, with mentions for many different parts of the car. There were rust fails in 2011 and 2016. And it's still showing rust advisories this year, suggesting there hasn't been a whole-car tackling of the problem - just patches. And the overall number of other advisories on the car is far above average, suggesting the owners haven't pro-actively maintained it.
But - whether it's this car or another one - there's only so much you can glean from reading between the lines of the MOT history and the seller's puff. Which is why you're thinking of getting a professional report. But, really, pro surveys are hedged-about with so much arse-covering that the take-away message from them is never clear. And most of the pro's are geared to look at high-value and relatively new cars: checking for ringers, undeclared accident repairs, excessive wear that indicates clocking, major drivetrain faults, etc. Rust is such a non-problem for modern cars that they're not really geared to look for it. But for buying a Mini, where the priority above all others is to find hidden rust issues, a look-over by an amateur enthusiast, or even a mate who knows a bit about old cars in general, is going to give you a more useful take.