OK, I have two of these. The first I bought for £500 and fixed up.
The R50 is the one to buy. Cheap, easy to work on. The engine is sound. The Midland gearboxes before 2005 are pretty fragile. The Getrag 5 speed gearboxes from 2005 onwards are much better but can suffer problems.
You can buy them cheap because people think the head gasket has gone, it may have done if the car was seriously overheated. Generally it's a cooling system problem, most likely the electric fan which gives no warning that it has failed until the car overheats.
The car is easy work on and well engineered - with the Tritec engine. Parts are really cheap and easy to get.
You will need to fix lots of minor issues like CV boots, front top mounts. PAS pump. If you can do that yourself these are brilliant cars, 45mpg on a run, allowed in ULEZ zones, amazing handling. They are noisy on the motorway though.
DO NOT BUY the 2007 onwards, Gen 2, R55/R56 mini, it had huge engine problems. The engines almost always need a full rebuilt after 50k miles. They suffer from cracked valves and failed piston rings.