I sold my 508.24 earlier this year primarily because of its age and my fear that if it broke, it would be expensive to fix or impossible to fix due to the lack of parts. I spoke to my local Meridian dealer, and he advised against spending any money to repair such an old unit if it broke (but of course he wants to see me new stuff). I sold the 508 to The Music Room and they listed it for over $900. I recall seeing some other on-line classifieds for the 508 in that price range. In my case, I mostly stream, so I just bought an inexpensive CD player (that matches my integrated amp) and connected it to an external DAC.
I think the practical answer is to just get a new transport if you are using an external DAC. If you like the looks of the 508 or have an emotional connection to it, it is a tougher call.
I'm curious about what external DAC you are using, because I thought the 508 was pretty damn good as a stand-alone CD player, even by today's standards.