Off the top of my head I can't imagine there would be any change from £500.00, possibly more. Speak to a specialist gearbox company would be my advice. Sorry, good luck.
I'd go 20/50 mainly because the gearbox is lubricated by the same oil as the engine. IF the gearbox used a seperate oil it would be 75,80 0r 90 grade so the engine oil in a mini has to be quite thick. The higher the number the thicker the oil. Agree with worn baulk ring or maybe synchromesh hub...