ok - now I've read through the etire post, and looked at the additional photos, here's my two cents:
1.) Unless you're looking at a 340 car or something that is desirable as it rolled off the assembly line, don;t worry about "original." An original 318 car compared to an original /6 with a 318 transplant will reflect very little difference as far as retail value.
2.) Any classic Mopar in the $5,000 or less range is going to need work. You have to consider whatwork needs to be done vs. what already has been done.
3.) Now that you know to expect a car that needs work, expect that it's not perfect and, most likely, will be somebody elses' incomplete project. so, in reference to the /6 radiator, or black engine bay - it's not really that big of a deal, and you need to take into consideration what needs to be done to finish the project. Now if you went w/ the green one (let's say it WAS local), you'd STILL have to do the quarters, and essentially that same amount of body work, but after that, you'd have the interior to contend with. Add to that the wheels, brakes, exhaust, etc. I think the purple car is a better starting point - and you can drive it and look good now compared to years from now w/ the green car project.
4.)The body work will have to be done, but all-in-all, I'd say that it looks pretty solid. I'd like to see under the trunk mat though.
5.) I doubt the seller is looking to lie or cheat about the K-member, I think it's more likely that he's just misinformed. that's why you do the research(like comming here) so that you are informed and don;t have to rely on what other might tell you.
6.) While I wouldn't say it a DEAL at $4500, I have no problem saying IT IS worth that much (IMO). Now I'm sure you can negotiate - especially in today's environment - and if you can get it closer to $3800, well, that would be even better.
7.)72 Duster is a much better looking front end than the 73 - and I own a 73.
negotiate the price, and go for it.