Neat gun, doesn't have much collector value anymore due to mods, but seems in clean shape. $1800 ish is about right. In which there's no way I'd sell a gun that was my father's.
Some good points... Even though it was refinished, whoever did it didn't get really carried away with polishing it...in most spots, anyway. Just judging by the pictures the edges and rollmarks still look fairly sharp. The thumb safety and slide lock lever appear to be correct though I can't tell for sure from the photos. It looks like the mainspring housing is correct. I'm not sure on the grip safety. I can't tell from the photos if the frame around the grip safety has been modified. Probably not since that grip safety appears to at least be the original style.
The mag release button, trigger, hammer and grips are not correct. It should have double diamond grips. Not sure about the spring plug. Obviously the sights are not original. Are there any markings on the barrel hood or on the barrel near the barrel link? It's hard to say from the photos if the slide has been modified for the sight install.
Personally I'd value the gun at $1000, + or -. There have just been too many mods and parts replaced for it to bring much more than that. But...you never know. Most collectors wouldn't even give it a second look unless they just wanted a shooter grade, WWI era gun...which is quite possible. And, it's still a great piece of history, regardless of the mods.
Thanks for the pictures. It's a nice gun. I'm with the others who say you should keep it if it has any sentimental value at all.
Edit: I meant to say... The 1918 guns were very common and are some of the most widely available for WWI Colts. That's another thing that lessens the value just a bit. I have one from the same year. The slide is rough on one side from improper storage. Aside from that it's all original/correct and looks amazingly good on the inside. I'm guessing it's worth in the $1800 - $2000 range. Again...you never know what someone will be willing to pay. Just for comparison:
I agree with JD8 as I would not sell it. But if it hits the classifieds ill play, because if the modifications were done before your dad got the pistol it has some old Smith heritage with me. Regardless if it is positively identified as work done by a popular Smith.