i currently have a true noname i got from a "everything in bin 20 swedish krowns" ~2 euro.
I have used and helped others use ~10 different external modems, not one was any trouble. atleast not the V90 kind, I sadly have no isdn or *dsl experience.
have got two internal modems working but they where oooollld, before the winmodem era, v34 or something.
sorry, wish i could help more.
why not ask if you may test the modem before buying, I was able to test my w-lan card on my laptop before buying (in the store).