Any particular reason you’re after a 260? The bellhousing on the M20 is different to an M50 and will be rotated by about 10 degrees, meaning your shifter and crossmember will need modification to work.
Plus e30 tax applies to a 260 whereas an e36/46 gearbox should in theory be easier and cheaper to find. (E.g. a 220, 250, various ZF gearboxes and a 240 from an e30 with M40)