I have to say that this discussion is unusual. There are a ton of technical questions that need to be reeled in IMO.
A UHD Blu-ray Disc player is the state of the art.
They run from the lesser expensive Sony models to the more expensive models like the Pioneer. If you don't have a $1,000 budget for a UHD Blu-ray Disc player, then you aren't getting a Pioneer Elite.
Stop calling it a 4K DVD player, that's NOT AT ALL the same thing. You will get thousands of results for upscaling 4K DVD players and they are not compatible with 4K UHD Blu-ray Discs. They literally take a DVD and convert it up to 4K. It's a translation of low quality to slightly better than low quality in a very shiny wrapper. UHD Blu-ray starts at 4K and may include several features that you may desire.
Those features are support for Dolby Atmos surround sound as well as HDR formats including HDR10 and Dolby Vision. If a UHD Blu-ray Disc player supports all of those things, then it is about as cutting edge as it gets.
If you don't have a fully surround sound system installed, then things like Dolby Atmos and other advanced surround sound options become nearly meaningless. Dolby Vision has less support than HDR10 and may not appear in as many products.
There also aren't as many UHD Blu-ray Disc players on the market as there are standard Blu-ray Disc players as well as standard DVD players. So, whatever you are looking at, make sure it is a UHD Blu-ray Disc player and make sure you are buying 4K UHD Blu-ray Discs. They are pretty well advertised on the packaging as such. Not sure if any place rents UHD Blu-ray Discs at this point.
Better players will have some faster processors in it so it will start discs faster, it will improve the audio some, support all flavors of discs and formats of audio and video. They will be designed with better cooling, quieter guts, and better mechanics for a longer product lifespan.
But, at the end of the day, if all you have is a good TV and TV speakers, then certainly a $100 Sony UHD Blu-ray Disc player is a VERY solid way to get going with UHD Blu-ray. Pick up a HDR 10 disc and pair it with an excellent TV and you will get top not performance that 10 times the price barely improves upon.