I'm appreciate all the advice. At this point I need as much resolution as possible to make big prints. I've found an affordable company that is willing to give me a very good price on drum scanning, Big Al's out of Minnesota.
Big Al's uses a Heidelberg drum scanner, according to their website, but they don't say what model. Most of those are limited to 10 microns scanning aperture, which limits the real hardware resolution. I've seen tests where even scanning at the highest listed resolutions, the actual number isn't even half of that, and worse, because it's interpolated in one direction, you often see stair stepping in those scans on diagonals.
Most medium format films just don't have enough information to warrant scanning above 4000 ppi, not to mention that most of the lenses don't either. You have to have a very sharp emulsion with the sharpest lenses shot at the optimum apertures on a very stable tripod. In those rare instances, you will see a small increase in resolution on the scan, which may or may not make it back onto the print. You'd be surprised at just how well a great 4000 ppi unsharpened drum scan interpolates up. In many cases it's just as good and less expensive and faster.
I have my own Howtek 8000 HR and have done extensive testing on all sorts of different films at both 4000 and 8000 ppi and I speak from experience. There are only a few emulsions that make any sense - things like Velvia 50 - sometimes, T-Max 100, Technical Pan, and that's about it. On every drum scanner I'm aware of that can scan higher than 4000, they all jump from 4000 to 8000 with nothing in between. The Howtek 7500 does 5000 ppi in hardware and drops in one step to 2500. And when I talk about the highest resolution scanners, I'm talking about those that have an actual 3.17 micron aperture to generate that rez.
You seem to think you need all this resolution because someone told you that you did. Or something. How big are you planning on printing, and on what material with which printer? I suggest having two scans made by someone that really knows their craft and has either a Howtek/Aztek 8000 or an ICG and making comparative test prints at full size - and not just small crops, as they change your perception of how the larger full print will look.