2 But how close together are they ?

Can you fit the same linear path on a tape that you can fit on a floppy (not limited to the standard 3.5") ?

I think if you don't put a lot of time into it and don't expect to really use it for storage, that it is a good learning experience. You can start out with 300 baud FSK... at that rate you should be able to get something working quickly. Then you can move on to higher rates and learn what the problems are.