Bill is right -- a cheap DSLR is a great way to learn. I would even recommend a cheap prime lens. I used Canon, so for me it was a 400D and a 35mm f2.0 lens, but I'm sure the Pentax would work just as well.
You ARE right about ISO (at least practically, though not technically). You can think of ISO as "sensitivity to light"
ISO, shutter speed and aperture work together to determine "exposure." There are generally set exposure values. See here (especially "tabulated exposure values"
Exposure value - Wikipedia, the free encyclopedia Moving up or down one exposure value is moving on "full stop" in exposure.
There's much to explain here, but much that is already explained elsewhere. I don't have time this am to type, but try reading explanations at the following sites:
Exposure: Glossary: Learn: Digital Photography Review
The Luminous Landscape Tutorials contents
Digital Photography Tutorials (this was one of my favorites)
And the ever infamous wikipedia.
Also, Bryan Peterson's Understanding Exposure should be a good primer on exposure.
One thing I do recommend learning is the full stops on the aperture. Generally, progressing by full stops, it goes: 1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22, 32. There's a mathematical reason for that odd progression, but I just memorized it by writing it on a card.
So, if I am shooting at ISO100 shutter speed 1/200 and f/8.0 and I want to lower aperture to f/2.8 (for less depth of field, but it also lets in more light), that's four whole stops (8 to 5.6 to 4.0 to 2.8). I would need to either boost shutter speed 4 stops (200 to 400 to 800 to 1600) or lower my ISO (100 to 50 to 25 to 12.5 -- well, can't do that on modern digital cameras) to maintain the exposure. Alternatively, I could attach an ND filter, or come back some later time when the light is less.
Good luck with your learning! Concentrate on exposure (and also the depth of field work you've been doing). If you master these, you'll get a lot of the technical side of photography down (now, as for the art side... well, I'm still struggling with that!)