You can buy expensive drilled and slotted discs that look cool but there is little point in upgrading the back brakes. Making them too good, simply activates the ABS, so you'll never use their potential stopping power.
Tarox full fat slotted drilled etc etc - £140
EBC do a pair of pair of plain for - £70
Delphi have a pair of standard rears for £20 (the same as used on lesser models at the front).
Rear pads are about £15 a set.
If you upgrade the whole car, then better rear brakes might be worth having, but I believe even then there's no functional benefit in more power at the back.
Talk to Shop4Parts for a set of discs and pads.
You will need a wind-back tool for the hand brake cam and take a close look at the sliding pins and caliper seals.
Caliper rebuild kits with seals and piston are around £20 per side. Slider pins are another tenner.
Someone on eBay is selling pair of 500 Abarth / Panda 100HP rear callipers for £200. Red powder coated, etc, etc.