well if your serious about it start here and see if there's a tape how to:
then look up the "tar" command, that will help with some insight
google for some homebrew computer sites and see what others have done
the part of converting data to sound isnt hard at all, whats hard is coordinating the data stream and the tape drive together
yep, commercial tape drives are very very expensive, thousand dollars roughly, the tapes themselves are cheap
There are cheaper tape drives for PC's but they usually arent very good,I had one for awhile, it was pretty slow.The best ones come with a scsi interface, that helps alot.