I think you're doing the right thing. Working on an old bike like this is a very specialized activity and you need to find the right old-timer to assist you. They're out there, but you have to ferret them out.
Take the $2K you were planning on spending to fix it up and find a more modern bike for your son. Put the RD250 in your garage and start working through the problems one-by-one on your own and on a relaxed schedule. Whether you do the work yourself or take assemblies and farm them out, you'll find that there are literally hundreds of folks on the net who know of specialized shops and individuals who will help you through the process, including many right here. It will be fun. The RD250 is a good bike with a lot of interest and none of the systems are complex or intractable. And it will be worth enough when you're done that you're not going to be pouring money down the drain.