Hello from a fellow '74 Overlander owner! Congrats on your acquisition.
None of your questions must be answered by someone who owns a '74 Overlander but since your post caught my attention, I will answer your questions with my opinions.
I wouldn't advise replacing the A/C with a Fantastic fan. The opening for your A/C is larger than 14x14 so you would have to do some major fill-in work. If the A/C doesn't work and you really want to remove it, I'd suggest removing it and then covering exterior and interior with panels. But, before you chuck it, consider that there's a reason why all campers (Airstream or otherwise) come with A/C units. You can have it replaced or repaired for likely less hassle than removing and covering.
As long as your electric brakes are working on all 4 wheels, then they are just fine. The same technology is still employed today on campers and trailers. Heavier trailers go to electric over hydraulic but would be a lot of $ and overkill for the Overlander. My Overlander has more than enough brakes with 4 electric drums.
I think the terms rub rail and belt line are used interchangably. The lower belt line is 1 inch width, the higher stuff is 1/2 inch. I've replaced sections of my 1" and all of the 1/2" stuff (actually, the 1/2 stuff wasn't there when I bought mine!)
The size awning you need is the length of the non-curved part of the trailer. Do you already have an awning rail present? It lookes like C-shaped rail with a flat extending from it which is riveted to the body of the trailer. If it is there, measure the length and that's how long you need. I'd measure mine, but I'm not at home this weekend.
You can research and get opinions all day long from the forums on hitches. You can go basic and get the typical weight distributor like I have where you have to hook up the bars every time and attach a friction sway bar. There's more advanced versions of a similar setup which have different methods for sway control (Reese dual cam). Then there's hitches like a Hensley or ProPride which runs you thousands but is the cadillac for control of weight and sway. You have to strike the balance between your wallet and your comfort level for towing.
I have the original steel wheels with original hubcaps. I would rather have newer alloys. I'd be very interested in discussing a trade with you regarding wheels / tires. I will PM you.
Don't hesitate to ask more. That's what these forums are for!