Which Vintage CD Player To Use As A Transport?

If you want cheap, I'd get any JVC XL-Z model. You'd have to use an RCA SP/DIF to connect to your dac. I used to think all transports sound the same and used an old Sony DVD carousel. It died and I put a JVC I'd had forever in and the sound was so much better I couldn't believe it. It's as good as the budget Cambridge a lot of people love.

Not recommended IMHO. Vintage means old tech, used and likely transport and laser wear and tear. Can buy a new cd player for $100 on Amazon. Quality, reliability questionable at best.

Is Bulletproof and $100 mutually exclusive?

I had an old Sony DVD player that sound great as a transport, just could not believe that thing sounded as good as it did. but ultimately like all cheap players it died. Splurgitate and spend $350 on a used Cambridge Audio CXC and more than likely save $ in the long run.

Get one with a digital output so a better DAC can be added at some point. Denon has worked well for me, but all can benefit from new drive belts and lube

No, it is not a good idea, based on my experience. I switched from Arcam 192 being used as a transport some time ago and the difference versus Cambridge CXC was enormous, since than I climbed the ladder higher, but this is different story.