2k for a full engine rebuild, including taking it out and reinstalling it in the bike is actually pretty fair, I'd say. It cost me quite a bit less than that on my RD350, but I did everything except the press work on the crank, and the boring myself. While there's nothing very hard about any of it, it is a lot of work. If you want to give the bike to your son, and have it be a learn to ride experience, and not a how to work on bikes experience, it would probably be a good choice.
If it were me, I'd be on the lookout for a ratty RD that runs, and swap that engine into it. They go cheap, but still, you'd have a 40 YO engine in there. I've got a very nice, very modified RD350 that will be up for sale soon. The bottom end, I rebuilt about 2000 miles ago, and the top end had about 500 miles on it. Why am I selling it? I picked up an old Guzzi that has a long list of needs.
Is your bike really a 76? In the states, there was no RD250 in 76, just a 400. There was a 250 from 73-75. 74's were brown, 75's white, I don't remember what the 73's were.
I wouldn't bring her home to Mama, but Mama ain't home tonight.