I don't mean to be blunt, but if you are just figuring out things like how to light up an LED as your first potential breadboard project, and can't yet identify the names of electronic components or the symbols for a dimmer switch on a circuit diagram, then the idea of assembling a DIY IDE or SCSI interface for your Amiga in the near future is pretty much unrealistic at this stage.
Again - I don't mean to sound harsh - but I think any other assessment would be doing you a dis-service. I do commend you for learning electronics, but it will take months or years for you to get to the stage you can assemble a complicated digital circuit like a hard drive controller.
As for the ZIP drive solution for your Amiga - yes, it's possible. But if you are going to try and find a SCSI or IDE solution to drive the ZIP drive, then you might as well be using hard drives or CF cards on that SCSI controller instead.
I have heard about the parallel port solution for ZIP drives, but I'm sure it won't be pleasant (i.e. slow).
ZIP drives, in their own right, were always semi reliable it seemed (google ZIP DRIVE and CLICK OF DEATH together).
Really, your best solution for mass storage on that Amiga500, is probably the clockport interface and a USB Subway card.
I know it's expensive, but it's much more reliable in the long run.
There is a cheap way for you to get a SCSI controller for the A500. You would need to pick up a cheap Zorro II SCSI controller card for an A2000 (A2091 or something) and attach it to the side port of the A500 with a "slingshot" connector or gender converter connector. The A2000 card will then hang off the side of your A500 and look fairly ugly (it will also be exposed and need to be supported with some DIY arrangement), but then, with a cheap SCSI hard drive from eBay you could have a SCSI hard drive setup for less than $50.
(see halfway down this page for picture)
http://paulq.org/amiga/a500tribute.htmlOh, you would also need to power the hard drive. An external SCSI enclosure or spare PC power supply might work for that. Again, pretty ugly solution but it would work.
The absolute cheapest way would be to pick up a cheap 486 or pentium I laptop from somewhere (don't pay more than $30) and use it via a parallel port cable or null modem cable to act as a file server for your A500. I'm thinking of something like Amiga Explorer or the old Siamese system. Essentially, the laptop would be like an external hard drive for your A500.