just a quick note, well 2 really...
1) you can use ANY modem, so long as it uses the serial interface, you can pick up a £5 secondhand 33k3 modem plug it in and off you go.. i might have a 56k modem laying around here if you want...
2) you can fit a 3.5" HDD into a desktop case easily, even the huge old style 3.5" HDD will fit, I have a very old very large 512mb IBM drive mounted in one of the A1200's I own, and it fits...