dinosaur ISO player...

What about an upgraded Oppo like this one (not affiliated with the seller).
https://www.audiogon.com/listings/da-converters-exemplar-tube-oppo-105-with-every-upgrade-2017-06-20-digital

Esoteric, Marantz excellent choices, if your thinking DAC, and want a transport that will outlive you, I suggest a Pioneer V7400 industrial DVD,
CD. Best kept secret, you can buy them on ebay around $100 bucks.
I just bought one for a headphone set up, really nice! The laser is rated for like 36000 hrs. 

I've had , let's say - "mixed" experiences with modded units in the past. So I was hoping to find something solidly engineered from the start.
The HDCD factor - are there DACs that will decode that? Any recommendations?
Thanks for your input, it's truly appreciated!!!! Audiogon is a resource of the highest quality!

What about an unmodded Oppo?  All the models prior to the current decoded HDCD so you could probalby find a nice 105 or something from someone moving up for 4K.   My 95 has been rock solid.  Even if you don't like the stock DAC you mention DAC isn't critical so you could pass the digital signal.

 ARC Ref 7 CD player. It uses the same transport as CD jukeboxes so if needed, parts won't be a problem. And it is a real deal at the price they sell for used.

 I listen to a lot of vinyl, maybe half the time. The CD7 does not leave me wanting. It is very smooth, detailed and holographic. Just a truly excellent CD player. I have felt no need to upgrade even after I upgraded my phono preamp. My only other choice would be Esoteric. But I really like the smoothness the tubes provide.

 I have been down the Pioneer DVD player path some years ago when my Theta transport died. I picked up the Pioneer DVD player everyone raved about to use as a temporary transport. It was just horrid sounding even through a excellent DAC. DVD players do not make good transports. I ended up buying a demo Ref CD7 and selling my Theta.