Al, one of the two original sets I had as a small child was headed by a 370 so they are special to me. I still have that set in excellent condition.
When purchasing a 370, I feel condition is important, no decal chipping, all number boards and horns present, no damage to the coupler bars and minimal wear. A top end 370 is likely about $100, in very good condition maybe $60 to $75.
Be aware this is a single motor diesel with no traction tires. Seven freight cars or four passenger cars are the limit it will pull and that is on level track. The more cars coupled to the engine the more likely it is that the lead car will uncouple from the coupler bar on the engine. To pull more than five freight cars will require undercutting the link coupler. Even then a heavy diecast car needs to be the first car to pull a long train otherwise the lead car will derail in the curves. I used a floodlight car. If a sheet metal chassis box car is coupled to the engine adding four more cars will cause the lead car to derail in the curves. This seems to be a trait of the coupler bar on the engine.
I have 2 of those 370 diesels. I learned the lesson Tom mentioned about what car to be the first hooked to the draw bar. That engine is a a 9 grade so it really never gets ran. My other one was not in that good of condition so I personalized it with silver paint and custom dry transfer lettering and converted it to KC's. The conversion wasn't as easy as it looks. Removing the drawbar and changing to KC trucks isn't cheap. I bought the trucks off eBay. The best I could do at the time was 25.00 a piece. The end guards need to be changed too. Portlines has them. It was a train show cheap buy. 35.00.
As Kenny says, changing a 370 to KC's is neither cheap nor easy. Then, when completed, the engine remains a single motor diesel with no traction tires. Now, if two 370's were modified for KC's then double headed, that would greatly improve the pulling power.