New Begining, Let me first say ,I am also VERY new to learning about narrow boats. I will tell you what I believe is true. Over plating, I believe is when new steel plating is added to the sides, bottom of the boat, due to wear or what have you. I read about a boat built in the 80's I believe that only had 5mm bottom plating, so maybe that was prevalent in the past. Seems that 10-6-4 is now the norm. As to , why are some older boats worth more, are you comparing apples to apples. Meaning are the boats "like"? And maybe you could look at it as a classic car can cost way more then a new car.
As for your question about cost of a boat between 10 and 20,000. I think you could get one in that range,BUT, it might not be what you want. Meaning, size, layout, engine, etc. How much work does it need? Lots of thing to think about. When I'm looking at an older auto to buy, I break it down in three parts, Exterior, Interior , mechanical . If all three need major repairs, I back off, you get the idea? Good hunting.