I have 2 choices, partially rebuild the rear brakes with new drums, shoes, springs and more. Or buying a dish brake kit like this:
DISC BRAKE CONVERSION KIT FORD 9" NINE 9 INCH REAR END | eBay
Buying new parts for the drumbrakes will of course also cost, but will be a bit cheaper than the disc brakes
It depends on what you want to do with the car.
If it's a cruiser, it doesn't matter much.
If you do decide to go for discs, make sure to use something for which you can get parts - calipers, rotors, hardware, etc - and that you know what those parts are - the vendor doesn't hide things from you.
What size wheels are you using? Not many rear disc setups will fit under a 14in wheel if you intend to run them, most will fit under 15in, some of the higher-end kits that use 12in or larger rotors are going to require 16in or larger wheels.
Any change to the hardware requires consideration of what you've already got (current front brakes and relative front/rear piston sizes, axle flange size) and what else you'll have to change (master cylinder/booster, proportioning valve, plumbing, parking brake cables, etc.)
If you were in the US I'd suggest snagging the rear-disc parts off a late Crown Vic 8.8, in kit form there's the Explorer rear disc setup available as a Ford Motorsport kit.
