Kombiven, you're in good company here..... Flash, Frustration, Mental Challenges and Not Having Enough Money are the 4 cornerstones and foundations for the majority of threads on this forum!!
It's a pity that you're waited so long without successful flash images on digital before coming here and asking for help. There are some good ways forward....
Firstly, you're well placed to get going with your "legacy" equipment. Lucky you, you have the one DSLR model that does the old type TTL, and you have a quality TTL flash to go with it. I am not experienced with film era TTL, but plenty others here are, but my assumption is that you need a lens with the aperture ring set to A. Does anyone know of some good guide available to working with TTL on the AF500FTZ and a camera like the *istD? There must be something out there that someone can point you to. The flash and camera manuals will of course also be important.
For your K-M and P-TTL there are a few budget options, eg the Shanny or Yongnuo (new), or you could look to a used Metz model. If you could find something like the AF-540FGZ used then you would have P-TTL plus Auto "A" mode, which will allow you to use older K or M type lenses . There's a section in my "Pentax Flash Guide" about A Flash mode and how to control flash exposure with it.
The Guide will give you all the fundamental knowledge you need to get going with a Pentax DSLR and a P-TTL flash and start getting good flash exposures and allow the creative control you had before but with your newer equipment. Click on the link in my signature below, or the link added above my the earlier poster. And keep coming back here to ask anymore specific things, plenty of supportive people will be glad to offer help!