Early versions had 1MB cartridges that were chassis-specific, as you described... one cartridge for the 124, another for the 140, etc. These were later replaced / superceded to a pair of 4MB cartridges which covered all models through the 90's. Problem is, locating the correct/late cartridges isn't easy. But if you don't need to work on all models, and you have the early 124-specific cartridge, that's all you need for the 124 / E500E.
I suspect a proper SSD makes a big difference in the C3 performance / boot time, and as you said having the EPC+WIS on the same machine can be handy. Given the difficulty in locating HHT's these days, it's far easier to just use the SDS. The SDS runs an HHT emulator called HHT-Win for those old 1990's models that originally used an HHT.
HFMScan had the ability to log data, I believe, which is generally not possible with HHT or SDS. This was the primary advantage of HFMScan, IMO, although not many people need datalogging.
