I take it you have done a leak down test on it that's how you know no2 piston rings are shot,
A Piston ring set from bmw are around the $50 ish plus tax per piston so you will need x4, and make shore you follow all the engine building basics glaze bust, ring gaping etc RealOEM.com BMW R56 Cooper Crankshaft-Pistons
I don't now if you have had the engine to bits yet, but you will need a load of other bits to ie gaskets, oils, filters, coolant, manuals, special tools. And the list goes on don't think its a Saturday afternoon job.
If you are going to take it on, change the cam chain, guides and tensioner to get the bits from mini the prices are not bad and if you order everything in one go ask for discount normally you can get 10 to 15 % if your nice
Check for carbon build up in the inlets to.
Anyway good luck hope it goes ok :-)
Hope this help