DE games are notoriously noisy when it comes to audio hum/hiss. You can rebuild the sound board but it won't likely help. They're just freaking noisy.
And your DMD being noisy (high pitch squeal) is just the DMD being old/going bad.

I can tell you the DMD noise when away when I installed the ColorDMD! Still have some speaker hum but I got used to it.

I had a Jurassic Park, all original boards, no hum. Played it for a year.

Then the power board died one day. I got a new rottondog board, installed it, and then I had the hum. Nothing I did would make it go away.

I don't know if this will help anybody or not, but there it is.

Some people have had success, myself included, "floating" the sound board off the metal backing with nylon washers and screws. You can get plastic screws at any hardware store for a total of about $1.50, so it's worth a try. There's a ground loop on some boards that is eliminated when the board is floated. The board will still be grounded through the connections.

Every DE game I've owned has a hum (including TFTC). I haven't dug into the issue, but there are folks on here that have ways of correcting it.

I have tried several fixes on various DE/Sega games.. The hum never seems to go away.

Look here: https://pinside.com/pinball/forum/topic/data-east-speaker-noise-ideas-for-a-cure/page/11

On page 11, Posts #505 and #506 explains how adding an additional small power supply completely eliminated the hum for those that tried it.

It does work. And many many many others have had mixed results with replacing caps and speakers and such. I would actually get the $20 power supply first and see if that resolves your hum. If not, go to other less likely causes like speakers, caps, amps, and grounding. I've floated boards and upraised speakers and it usually does not completely eliminate the hum.

If you're not electronically savy, you may need to call in an expert.

Also, I think you are overreacting a bit with your post. It does not sound like the seller deliberately mislead you about anything. This is a VERY common problem that can be very difficult to resolve. Seller likely had no idea how to resolve it either.

That won't fix the attract mode buzz though. So far, nothing that I have ever seen fixes that. I have most other buzzing stopped at this point.