Sometimes a problem like this seems like a major ordeal but usually even novices or first-time owners get things going, with help from others. On the plus side you'll learn much more about your machine and be better informed on how it works. Even the slot people in casinos had to learn when they first started out. Hang in there, most likely all will be fine soon.
From what I know the Door Open M error is a reference to the Main door (instead of the cash box door or the door to the overflow compartment in the slot stand). The Door Open M error is often caused by either faulty door optic sensors (bad or misaligned), or door switch. And wires to these parts can be broken or pinched also. Obviously the casino doesn't want the machine door to be open and allow access to money by unauthorized people so they require that type of sensing. It can be a pain to home owners until resolved.
If you happen to live near a slot machine dealer or service place they could probably help, you'd have to call to see what they could do and what the charge would be. Also, if someone in your area has a similar machine they could help by trying your I/O boards in their machine, or their I/O boards in yours. And you could send the boards out to a shop to have them tested but that might be more cost than just buying a good used board for a lower cost.
You could post your general location here (be careful to not say your exact address except in a private message) and if anyone on NLG was near that city they might offer to help. To contact a person privately look over to the left of one of their messages and click on "Private Message".
To identify the I/O board specifically (and any other circuit board) you can look for an IGT part number written on it somewhere, often near one corner but can be anyplace. You can also post a picture of the board and others here can help id it. That way if you decide to order one you'll know the correct board to get.
Also, I looked back thru your messages but didn't see exactly what your machine model is. Can you post a photo of the entire machine from the front and also the idplate (usually on right side below handle) that clearly shows the model number? That will help others to make sure you get the best help.