I played around with both the Pentacon Six and Kiev 60. They are the same 6x6 format with nearly identical operation. Many years ago when pros started going digital I scooped up a few of both. I mostly used them for astrophotography, but I did other stuff too. I generally preferred the Kiev bodies, but the Pentacon lenses seemed better. I say "seemed" because it was difficult to tell the difference when dealing with film. Both would be available on
eBay like you mentioned.
While I will agree that manual cameras are great for forcing someone to learn the foundations of photography like focusing, metering, depth of field (and preview), etc, now that I'm older and have a little more money I would NEVER go back. I do appreciate what I've learned from them, but even more I appreciate what today's digital cameras can do. I've since sold all my medium format gear.
Keep in mind film is getting harder and harder to find. When you run out of your stock, you're done until you can find more, usually online. Developing is getting harder and harder to find, especially medium format, so you'll probably have to send that away too unless you're lucky enough to live near some pro lab that still caters to film.
Today's full frame 35mm digital bodies have more resolution than 6x6 film and more ISO capability. I would dare say that a high resolution prime focus lens on a good crop body would also compete very closely with 6x6 film.
But by far the biggest reason I can think of to discourage you from doing it is the instant gratification provided by digital. I can't think of a better way to see the results of your settings immediately. When I was learning with the manual film cameras, it would be so long between when I took the picture when I looked at the print that it was difficult to remember what I did. Now the camera records all the setting right with the image. Plus it costs nothing to develop the pics. Of course printing costs the same, but why print? No reason to waste money on that.
Without knowing your situation, my suggestion would be to buy a DSLR and turn the dial to full manual. Then buy some prime focus lenses (not zooms) and flip the switch also to manual. If you go this route, you would need to get the camera altered by installing some kind of a focusing screen more suited to manual focusing. With this scenario when you finally decided to upgrade, you wouldn't have to start from scratch. You start by buying one better lens. Then another. Then a better body or flash. In the mean time you're still shooting and never miss a beat. Good luck!