What you need:
1. Set of R Calipers, they sell on ebay between 300-600$.
2. Brake pads, (x4) make your choice, OEM, Aftermarket, Subaru STI pads work (Front only). Rockauto.com
3. Rotors (x4) Rock Auto
4. Mounting bolts M12x30mm Hardware store, use only Grade 10.9 Bolts.
5. Brass sleeve 1mm thick and 14mm diameter. This goes in the hub where the M14 bolt went for the stock caliper. New one uses M12 bolt so you need something to fill in that 1mm gap.
FR: 336mm Rotors
M14X35mm mounting bolts.
RR: 305mm Rotors
S60/V70 R Brakes
FR: 330mm Rotors
M12x55mm You will need 30mm since the steering stops will not be needed.
RR: 330mm Rotors
M10X30 Reuse the stock XC90 bolts.
You will need some sort of wheel spacers as the stock wheels sit way too close to the calipers and the Brembos are quite a bit larger.