I have two. One standard model 18 and one 18C compensated model. I want that 100rd mag, er 5 or 6 of them.
My limited experience [around 3000 rounds] suggests that the 18C is indeed the way to go. The custom tower for the Beta-C MP5 magazine is a preproduction item [or was] but sufficient nosing around at the subguns.com website will put you in touch with the developer.
I have little doubt in the ability of the Glock 18 to reliably chat away at full-tilt, but the drum magazine, which needs to be scrupulously lublicated with the graphite included with the drum in its ammo pouch, is probably the weak link of the system. Not to worry: the high capacity 33-round Glock 18 magazines are stone reliable, and reasonably cheap, in the $20-$30 range, now that the infringement of the high-cap magazine ban is hopefully forever in the past.