Hello! My project car 1999 Ford Fiesta 1.8D, has bad rear brakes. I plan to remove the tyre and hit the drum until it breaks so it can be removed and moved again. I also have a bad experience with drum brakes overall, and would love to upgrade this dangerous type of braking system to disc brakes. I had an accident with a Volkswagen Passat B3 where the passenger side rear brake got jammed after intense braking, and the car jumped to the right side of the road fast. New winter tyres on the rear wheels were destroyed because of the drum brake lock. The right side tyre was much worse. Drum breaks are faster to get stuck if the car is not used long or short period of time. Another reason why I would like to convert is to get experience for my newer Fiesta to convert the 2008.November version as well. The 2008. November version breaks are enough to drive, but seem weak on higher speeds. I think rear drum replacement could fix braking performance to make it safer for higher speeds. I think the current breaks are good for 50km per hour, but above it struggles a little. Has someone done it, or is it still theory? I heard that my newer model is kind of simpler if I find the ST version full rear system break, so I can convert mine. ABS Electrical might require coding in the ECU or other electronics to make it work. 

Plenty of people have upgraded to rear discs on Mk4 Fiesta & Puma.  None of them had discs as standard though, so you need them from another car, Mk1 Focus was a common donor car.

Mk6 Fiesta is easy, as you say just need the parts from an ST150.  (Although personally I'd swap the whole car to ST150!)  Edit - Just seen your other post, the 2008 is a UK Mk7, not Mk6.  That is the changeover year, I had a 2008 Mk6 myself.   Mk7 Fiesta disc upgrade is still just as easy though, only needing the ST180 parts, as you said.

Sorry, chum drum brakes are very reliable and hundreds of millions of cars got to the scrapyard without the slightest problem with them. Disc rear brakes on a 1 8 diesel Fiesta are completely unnecessary. I find their fitment on my 125 HP mk8 to be less well balanced than the drums on the 100 I had before. It is overbraked at the rear. Still it is your money and your " I'm going to hit it until it breaks" mentality suggests that listening to reason will not be high on your methodology.