@Daniel D., if you are under contract, you don't need any letters to send to the sellers, just get your GC to do as walk-through with you and tell you what you need and what permits will be required for it. If the walls you want to remove are not load bearing, you don't need a structural engineer for this project. Usually, your GC will pull any necessary permits but you can do it yourself if you want to save some money on your project. If you want to pull your own permits, contact your local building department once your GC tell you what permits you'll need and they will tell you how to apply for them.
Architects are useful if you are doing a full gut rehab or building custom homes. If all you're doing is moving/removing some walls and maybe adding a bathroom and finishing the basement, you don't need an architect.
As far as what items to look for in your due diligence...again, this is where your GC will come in. And anything you see that looks odd/weird/fishy, show it to your GC during the walk-through and they will tell you what it might be. There are far too many items to look for to list out in a forum. Since this place is in NJ, humidity is a factor, much like Maryland so you'll want to look for any discoloration (i.e. mold), especially in the bathrooms, under the sink, basement and attic. Look for carpenter bee holes in wood and termite damage. Look to see how many Amps are going to the electrical box. You want at least 150 amps but ideally, 200 amps for a modern house. Windows should be double pane, not the old, aluminum stuff. The aluminum stuff needs to be replaced. Look at the furnace, HVAC and boiler to make sure these items are in proper working order. If the house has an A/C unit, check it as well. These things can be old but should be in proper working order.
Roof should be inspected as well. If the house has a basement, look for water stains on the basement walls or where the wall meets the ground. If you have water stains there, chances are, the house will need an indoor or outdoor french drain as you have water seeping through the walls and/or where the wall and slab meet. Also, if you do need to replace windows or doors, measure them to verify they are standard size that you can easily buy without having to special order all the windows and doors for the house as this will drive your costs up significantly (trust me). If you can remove a light fixture from the ceiling in a room (dining room is usually easy since there is a light above where the dining table should be) and see what kind of electrical wiring is used, this will be a great help. Depending on the age of the house, the wiring might have to be replaced so seeing the wires and their condition will give you and your GC an idea of what this will require. Re-attach the light fixture back how you found it after you are done inspecting.
I can go on and on but like I said, you can't capture everything in forums. I'd recommend picking up J Scott's book on flipping houses. He does a fantastic job of taking you through the big things you need to look for in houses you plan to flip.