Been here, b58's are probably still like 10K extra over the N55 at minimum, Im just guessing but thats how it was for me when looking.

A clean EWG N55 M sports in my area in the US go for around the same price, if not more still. Honestly Im not sure where people find clean n55 m sports at $13k, under 100k miles.

Otherwise, it depends what you want to do with the car. Do you plan on tuning it? You want to keep it stock and daily it? If you want to tune it, 400 hp is cool and nice, but honestly I would just save for a b58 at that point, you can get more power and dont need to upgrade the intercooler.

But also, if your at that point, there's a chance you can find a not bad f80 M4 for not much more.

Main things would be to lift it, inspect for rust/worn suspension, look at the transmission, maybe do a compression test to check engine. See if breaks/rotors and tires have been changed recently because it can add up quick. A solid shop's PPI should cover it all.