Even a post-WWI rebuild M1917 will have value so if it checks out otherwise $200~400 isn't too bad.
At one time M1917's were the redheaded stepchild of the collectors world. Yes, they had history, but they just weren't that desirable and they sold for cheap. Now with '03 Springfields bringing $800 and (way) up, they have come into their own as a US military collectable and the prices have started to climb.
Original condition M1917's are getting hard to find and will bring a premium. One thing about the US Endfields is that they have the manufacturers mark on all major components and are therefore much easier to verify their originality.
As for the barrel, you would be surprised what a good cleaning will do for it. I have had great success with using a bronze bore brush, Hoppes No9 and lots of elbow grease. Clean it from the breach end and after a half dozen passes with the brush, run a couple patches down the bore. It may take a while, but I have had barrels that looked hopeless come out nice using this method.
I once bought a 1898 Krag that was in beautiful exterior condition very cheap because the barrel looked like a sewer pipe. It sat in my safe for a year while I looked for a barrel for it until one day out of boredom, I decided to try and clean it up. Due to the difficulty of trying to push a brush through by hand, I wound up having to put the brush on a cleaning rod in my drill and running it through the bore (slowly) while chunks of petrified carbon came out the other end. It took a while, but eventually I was rewarded with an excellent shiny bore with very few pits.
Just for a little gun related eye candy, here is a Winchester M1917 that was overhauled at the Ogden Arsenal during WWII and inspected by Elmer Keith.