If you buy them new and handle them properly, you shouldn't need a cleaner.

If you're buying used disks or fingerprinting them, use a soft cloth, like a eyeglass lense cloth, and move from the inside to the outside. Don't clean in circles.

Don't know of any scratch repair kits that work well.

As far as a cleaning machine, even the ones in video stores don't work well and will probably be a waste of money.

A Blu-ray player will play both. A 4k Blu-ray player will play both and 4k disks.
A standard DVD player won't play Blu-ray disks or 4k.
The price differential between a standard DVD player and a Blu-ray player is very little. Buy the Blu-ray. It will also upscale standard disks; providing you, in many cases, a better picture from a standard disk.
The one down side to a Blu-ray player is the cost of Blu-ray disks are higher but not significantly higher.

I had so many compatibility problems with the K8500 and my LG 4k HDR TV that I called Samsung k8500 technical support team.
I asked one question and got this exact response. "Have you had compatibility problems with the k8500 and other than Samsung TVs?" The response was immediate. "Yes and it may get worse."
I then called Best Buys Geek Squad and was told "We've heard of this problem before."
Then I called a Best Buy Magnolia location and two salesman, one owned a Sony 4k TV, said they had issues with the K8500.
What you're experiencing is probably caused by compatibility problems inherent with the K8500.
Where you really find significant compatibility issues is with your picture settings. Go into each of your picture settings then go to the picture options and/or advanced picture settings and see how many are greyed out, meaning you can use them.
I have more than 20 picture options that I can't change and LG has told me I should be able to change them.
The K8500 has already resulted in the return of one TV and the same problems with a second. I'm awaiting a new mother board. If that doesn't work, Best Buy will be getting another visit from me.
Oh, after Samsung admitted to compatibility issues, Best Buy took the k8500 back and applied all I had spent on it towards the Panasonic UB 900. I've had no additional compatibility problems since I installed the UB 900.
GOOD LUCK.

I believe there are nine picture settings on your TV with at least 10 to 15 more within each of the nine main settings. If you don't want to fully adjust each of those settings, use the nine.main settings as they are when you bought the TV. Some are set for cinema, sports, games, vivid, etc. You should try each one with the type of programming being played and then determine which one works the best by program type.
You have another option, one I'm not in favor of, ask Best Buy to calibrate your TV. They charge more than $200 for this service but claim it will give you the optimum picture.