Which Vintage CD Player To Use As A Transport?

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.

Find an old Pioneer Elite and do anything you want with it. They sound great either way and are solid as they come. I have 2 PD65 and gave one to a dear friend years ago and they all still work flawlessly. Before my Marantz player, I used one as a transport into a Mac C50 and the platter was quiet as a mouse. It will be more than 100 bucks, but good god my brother, your front end is super important, and you get what you pay for. Save for a while and buy something in the $500 realm. It will be worth it. Or not. Enjoy the music either way.

I am using a Rotel CD 591 that was gifted to me. Using the Digital coax to a Cambridge Audio CXA 81 internal DAC. It's a bullet proof transport. I do use the internal DAC of the Rotel on the very few HDCD's that I have. I have also had positive experiences using used, lower end NAD CD players as transports. They sound excellent until they ultimately expire from faulty file reading circuits. Also count on burned out displays.

An Onkyo C7030 makes for a nice transport.  Several years back they were selling  for under $200 on Amazon.  I paid $173 with a four year extended warranty. Under light use it's still working fine over six years later.  Perhaps you can find a used one for $100. The C7030 is a very nice sounding CD player that also works well as a CD transport.  It has both optical and coaxial outputs.