Which Vintage CD Player To Use As A Transport?

Not a good idea because the dac outputs on old CD players leave a lot to be sonically desired and/or your replacement dac won't have the old outputs

Not sure what tweak1 is talking about. Plenty of old high end dvd players from Sony, Pioneer, Toshiba, etc make great budget transports. They all have coax/optical outputs which all modern dacs have. I personally have a Tascam Cd200 which I picked up for $80. Nice transport. Another good choice is Sony s7000 or 7700.

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.