AMD One Click will give your Mini One the same power as a Cooper (exactly the same engine anyway) and let you switch between standard and non standard really easily.
I think if you drive the Cooper then the Cooper S you will be impressed with the difference - more power, much better gearbox, addictive whine of the supercharger, and it pops and crackles on the overrun
What you want is a post July 04 model if going for the S as it had a mild facelift - new lights front & rear, different grill surrounds, new 3-piece dash (instead of 5 piece to apparently cure rattles), 3 spoke steering wheel, different shaped armrests on the door cards, digital speedo reading on computer, etc.
Mechanically the engine saw some changes which released a few BHP taking it up to 171bhp, the standard exhaust now popped n farted the gear box was new with shorter ratios, and in jan 2005 you u could add a LSD for the first time.
If you need any more info just give me a shout.