http://www.e9coupe.com/forum/threads/rear-disc-brake-conversion-on-1970-2800-cs.383/
Answers to your questions are very subjective. I have never given the upgrade much thought for an E9, so my thoughts are as a civilian observer. I am mildly familiar with the 2800 drums and have dealt with similar versions on 02s and other drums systems. Do you have all of the parts to perform the conversion?
Without fully knowing what you have to salvage versus what you will need to obtain from a donor vehicle, only you can answer your question.
Subjectively, there is nothing particularly wrong or bad about the 2800 drum brakes. Comparison with the disk setup is somewhat unfair. The disks tends to be superior because of greater swept braking area, they are less susceptible to over heating and there are probably more friction material choices. However, the drums may actually be lighter (I do not know this as a fact), and for the most part, your car's front brakes do most of the work. (They do tend to require slightly more maintenance and adjustment - than the disks. But the maintenance tends to be relatively minor unless you put a lot of miles on the car.) (A tiny advantage for the drums is that they can be adjusted to provide less drag. Less drag = greater fuel economy. They may be "less modern" but until recently, they were still used by a few high fuel mileage vehicles, e.g., Prius.)
Unless you are a hard driver, and do a lot of downhill driving the drums are adequate for average driving situations. Another consideration might be the originality of the car. Some upgrades are not welcome by hard core purists. Others could care less.