I have one of those 6400's myself. Nice Mac, arguably better than the 6500.

Mb will probably be fine after giving it a good cleaning to get rid of the battery deposits. A dishwasher run may be a good idea too.

The FDD in my 6400 is a bit worn too - it struggles to eject disks. I'm thinking that the drives in these cases may get rather dusty for some reason, and need cleaning more often than other macs. This problem may also go away after cleaning.

AFAIK you need at least OS 8.0 to run a G3, so keep that Sonnet card out for now.