dodgy pot and 250

i couldn't tell you if it is the pot or system...but pot would be more likely. assuming its a regular pot it would be easy to replace, just getting to it would be the main task.
otherwise all fx parameters are assignable to whichever pots you wish (or the d-beam), so if you can live with just 2 pots, it is easy to work around the dodgy one.

i have two 808s, one with 750 drive, one with 250.
the 750 is fine for use as a hard disk recorder, but it is not so good for processing samples, lots of extended waiting. ive had no probs, but others tried it and quickly switched back to the 250.
so, you are probably better off with the 250 drive, it operates as well as the 100 but with more memory.
imho - iomega is best, for drives and disks.

i dunno european prices but 400euros seems quite high. here in japan, used classics on 100mb go for US$150.