I'm set for the controller adapters, I have 2 D9 adapters made by the same person who makes the Ultimate PC Interface. The supported devices are almost identical between the 2 adapters, aside from the 2 Intellivision devices that require both joystick ports (the typing and musical keyboards).
Yes, this was the conclusion I came to as well, but as you mentioned they are going for quite inflated prices on ebay right now. So I'll just have to keep an eye out I guess.
Anyway, thanks for confirming this setup works well.