Welcome to our forum MicJoe. Your quarter panels seem very fine on the pictures. Here in Canada all you need is to fill the holes with bondo to pass the inspection but I don't know how it works in Connecticut.
If you have no other choice than having the quarter panels replaced you don't need a metal fabricator unless it can be done cheaper depending of the damages. Quarter panels are pretty cheap but you will need a welder to replace them.
The quarters are the panels located on the side of your car between the door, the bottom of roof, and the tail light pannel but sometimes all you need is a section of this quarter panel. Once the panel is removed you might also find more damage behind it and if the rust is not removed everywhere it will contaminate your new quarters so the bill can be very expensive if you want the job to be done right. I also noticed the dust on your carpet that looks like rust so I am afraid your nice Mustang is hiding a lot of damage.
If you can send us some pictures of the engine bay, trunck floor and underneath your car we might be able to tell you more about its condition.
Here is a link for reproduction panels but you can find them at any Mustang part store:
Mustangs PlusI wish you the best.