TC and others are giving good advice about value and condition. I would never buy a vehicle with no title except for parts. NEVER. No matter how many times someone says "all you have to do is..." NEVER! Most older trucks probably would not be in danger of being stolen, but many states make it very hard on you to get a title. I just don't want the headache and risk. At least try to check the VIN at the DMV before you buy. A bill of sale is just a receipt.
If you have plenty of space and an understanding significant other and it's pretty solid except for missing parts, it sounds like a great buy. Then buy another with a title and good bones and build one out of them. The left over parts will recoup much of the investment. You get a complete truck. I have done that. Remember, with missing parts, you will be just about doing that anyway. Looking and looking and dealing and dealing and bringing home parts to put in a pile and so on. Buying two is a real good way to go....or three. I have redone many, many, suburban's and panels, that's all I do. I can't reiterate enough what has already been said about availability of certain parts. One with a good body behind the doors is worth way more than a good body in front. Another tid bit: If it only has one seat and that is a rear seat, that seat is worth what he is asking for the whole thing, upholstered or not. Good clam shells= same. Good Barn doors=same. Clam shell swinging tail light= $$. good cargo floor= $, no dent top= $, good bumpers= $. Didn't read G-mo's tip, so sorry if I repeat. He is a good guy and a great resource.
IMO: upgrades are down the line, get the truck(s) first. You'll have plenty of "upgrades" to worry about after you get them home.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn