Another info gathering exercise for me. I am going to change the drum to disc but wanting to know (if there is any) which setup is better 7.5 or 8.4. At this stage i know it's the case of wheels your going to run ie 7.5 for 10" where 8.4 is too big so you go to 12" but what else? also which kits/brands are recommended?

Pick your wheel size then use the factory AP brakes.
Simple and about as cheap as it can get.

My preference would be 7.5" discs with 10" wheels...which are likely what your Mini came with if drum brakes.

Mind, if I was installing a fire breathing, earth consuming power unit, I might like to be better able to stop...so, 8.4" discs.

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Select a gear, bring up the revs and release the clutch...off you go!