If you haven't already, look at all the bearings, bushes and mounts all over the car. Maybe some engine stays or gearbox stays. Do you have a heater? and what type of engine cooling system are you running?
If your happy with the brakes and suspension I would be looking at all the little things that hold the rest of the car together and in place

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Respect mine and I'll respect yours.

Here ya go, this should do it:

http://www.minisport.com.au/prod909.htm

Or this: ooooh lightness! (wtf?)

http://www.minisport.com.au/prod1134.htm

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

I'd be spending the cash making the car easier to own and generally nicer to drive.

All door rubbers etc all ok? Electronic ignition? Fix the wonky speedo? Or, prevent something going horribly wrong and replace the (probably) perished rear brake lines? Seat trims you said ok, but are they comfy?

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Cheap, Fast, Reliable. Choose two.

geez 12grand not bad lol