STOP dont buy a tower or anything like that.
All you need is a 2.5" IDE hard drive, and you could get away with the 3.5" one you already have. You'll need to take the computer to bits a little to fit it, but if you are happy to do that, all you will need are one or two things.
To use a 2.5" drive all you will need is a 40-pin IDE cable.
To use a 3.5" drive you will need a cable which converts the 40 pin 2.5" IDE connector in the Amiga into a 44 pin 3.5" IDE connector AND A floppy disk power splitter with Molex connector. 2.5" drives get their power down the ribbon cable, 3.5" drives need a power connector.
Also you will need to get hold of a copy of Workbench disks (especially the installer disk). Check to see which kickstart you have (3.0 / 3.1) and then get the right one off ebay or wherever.
To start with, the best tool for you is Amiga Explorer
http://www.amigaforever.com/ae/It's available on it's own or as part of Amiga Forever.
Once installed on the Amiga, using the serial cables you already own, you'll be able to write your ADF files back by simply dragging them files to the DF0: icon in windows.
(It will take a long time 10mins+ to make a single disk using just a serial cable)
To get Amiga Exporer onto the Amiga you'll need to be able to boot to Workbench 3.0 and use a 720kbyte dos floppy disk in windows. You must have done something similar to get AMI-PC to your Amiga.