Will look forward to hearing more of this project, Phil! Nothing much under £30K though (albeit dealer prices)
https://www.carandclassic.co.uk/car/C1271528 looks to have some upgrades (air filter, exhaust) but not others (engine cooling kit)
https://www.autotrader.co.uk/car-details/202009033246427 looks nice.
https://www.pistonheads.com/buy/listing/10757430 also nice

M3W5sp 2015, MSCC, MTWC, Oxon UK