I did this change on my SD. You need:
- 294mm discs - can be drilled though I think that's more fashion than function;
- brake caliper carriers from an S (294mm discs), not a Cooper/SD (280mm discs);
- new S pads;
- a new brake wear sensor.
That's it, the total. These will fit under most 16" wheels - see this very site's Wheel FAQ
for which OEM wheels fit.
The callipers remain the same and are not even disconnected. The dealer labour cost to make this change should be no more than for disc and pad replacement - there is absolutely no more work to do this, as the caliper carriers are removed during a disc change.
The end result is safe, in that you no longer get brake fade if you drive hard. I wouldn't say the 294mm brakes feel any better, they just don't suffer from fade.
If you haven't been suffering from fade (the brakes working poorly after big stops from high speed) and you just use the brakes a lot, so wear them out quickly, I'm not sure that the 294mm discs will be a big benefit to you.
My pictorial guide of dong this conversion yourself is attached - due to forum limitations, this is a low-res version but that doesn't have much effect.