Old plymouths are a ton of trouble to find parts for (compared to chevys and fords) and the 4 doors arent worth much of anything.
www.manheimgold.com is a site that keeps track of auction values... most you could get for the car realistically is 4500 bucks, and chances are thats after 10-15k in restoration unless the guy has an incredible car and is unaware. Rear quarter panels are expensive to do (assuming you can find them) figure on 1500 bucks for the labor in replacing it, and that assumes the panels behind it are good.
My first car was a similar investment, although it was a 1972 cutlass 2 door sport coupe (442 knockoff owned by by grandfather since new) which needed no serious metal work, had the rare 4bbl, dual exhaust 350/TH350 with every factory option and sold for 8k after about 14k in work.
From my experience it is better to find the more rare, more desireable project car that costs more... ie if you love the fury, then wait for a convertible that costs 5k to come along and spend 5k on getting it driveable, instead of dumping 10k into a 900 dollar car. However if you are looking to part it out, or for a good demo derby car go right ahead but the price tells me there isnt much to this car. Seriously consider what it is going to cost you and what you want to do with the car before you jump into it... convenience is not a reason to buy an old car... trust me.