If your taking it to a mechanic for review, he will know what to look for.... mechanically at least. You have to get some assurance that the truck runs well, without vibrations at speed. You should indicate that you want as much time spent on reviewing the condition of the FRAME from one end to the other and also the body for any corrosion issues.
While the body does not "rust", it can and does corrode, specifically where dissimilar metals have been required to meet. While companies like Grumman did a great job isolating dissimilar metals with plastic liners, over the years, those liners get squished out and slid out from between the metals. One area of focus should be where the body has been mounted to the frame.
Regards the Frame, you need to really look at what is left of it after it has been standing. Depending in where it was standing, it could have suffered a lot of damage from moisture that simply could not dry out. Study in particular, all mountings to the frame... suspension mounts, shock mounts, rear most portion of frame for thinning. Check the entire front suspension for rust problems. The front control arms often get thin over the years if it is an independent frt suspension.
If the frame is scaling, it does not necessarily mean it is beyond hope, but you will want to get under there with a needle scaler, knock off anything loose and get it protected with something to preserve it. I tend to coat it up with Fluid-Film rather than paint, because Fluid film will sneak into all the cracks whereas paint tends to just leave plenty of cracks and crevices for water to lay and cause rust.
Body wise, take a flashlight and study it from underneath as well as inside as best possible. Check the drivers floor area because it was often a steel pan and rusted considerably being in the drivers side wheel well. Look at all the door hardware, specifically the door sliding mechanisms. Rough rollers are replaceable, provided someone did not build too much inside because you need to slide the rails out the back of the cavity. Check for loose rivets up near the top edge all around the roof. Wind buffeting over the years sometimes caused those to fail.
BODY DAMAGE is also important. The aluminum used on these trucks is very difficult to straighten if bent up. Some replacement body parts are available, but expensive to purchase and replace. Look to see if the PO has put a few too many holes thru the body to mount things. Its totally unecessary to have done such, but it seems a lot of prior owners take the shortcut of drilling a hole in the body for any hook or wall they wanted to add. Fixing any holes requires someone to tig weld them shut, something a bit more pricey than just your average wireweld and grind repair.
The good news is practically everything you need to keep this truck on the road is not only available, but also affordable.... well, accept when it comes to diesel repair if you ever need it. That 4BT is an excellent engine if it is in good shape, but where you can buy a complete new Reman Small block Chevy 350 for $1500.... you'll spend that just repairing a turbo for one example. Diesel is only "Cheaper" when it comes to potential MPG's...... but only if its all running well.
The transmission is likely the bullet proof TH400 with a special adapter. Sure, you CAN break one, but it costs far, far less to rebuild than practically any other transmission.
1977 means you can get collector or hobby plates for the rascal as well. Love these old trucks if they are in good shape. They are really simple and easy to work on and You can keep them on the road forever with a little maintenance.
