The CD (or disk player's) transport quality is a rather important factor. Whether the digital processing is, or not, is dependent on the specific gear involved. For example an external DAC can improve many disk players, making the built-in processing less important.
I believe that it is important to have the ability to read physical media, so personally it's my opinion that everyone should have a disk player of some sort. Whether that's a standalone CD player or a DVD player or a unversal format disk player is more a matter of personal choice. Certainly the highest quality players tend to be CD only, but that doesn't mean you need something along those lines in a good quality HiFi system. As far as bang for your buck, it's often not one of a CD-only machine's advantages, and a more versatile multi-format machine would almost certainly offer better value.
If your chosen disk player is a computer-attached device, the one thing I would suggest is you don't play disks directly from it for serious listening, but instead rip the original to your computer storage, and use that file for playback. It of course doesn't matter for casual listening, but if Sound Quality (SQ) is important in a listening session, I know of few instances where that won't offer the best option (note I am not suggesting a standalone disk player wouldn't work well; I'm only referring to situations where your computer is the only disk player available).
With digital playback devices, it's extremely difficult to make generalizations about sound quality and price to performance ratios. Everything pretty much has to be considered on the basis of specific components and how they perform, offer the features you demand, how they fit your overall system quality level, and of course what they cost. There certainly are bargains out there if value is your top priority, and in some cases that extends to surprisingly inexpensive units.
I'm sorry in that I can't offer you any hard and fast rules, but that is the reality. If you have specific questions about a particular unit or two, start a new thread and I'm sure you will get plenty of responses to consider.