From a financial POV putting money into any boat is a non starter. Whether you buy an old boat for a few thousand dollars and spend 50k fixing it up or buy brand new and take a 50k hit when you sell it, that 50k is gone all the same. The economics of boating are enjoy the money spent because you are putting a match to the money once it's out of your wallet. This neatly explains the depressed condition of the boat markets today. In good times people can afford to burn some decresionary income. In bad times there is no discretioanry income to burn.
That said, there is good reason to find and fix an old plastic boat. For the love of the machine. While it doesn't make economic sense to fix up an old boat, no boating makes economic sense. You buy an old boat to fix up for the same reason someone buys a new boat. To enjoy it.
There's a few other reasons to fix up an old boat, - for the fun of it. That's assuming of course that you like doing that sort of stuff and have the time to do it.
Another is that you get to choose how it's fitted out. Finally when you're done you're going to know that boat like the back of your hand. If something breaks down, you'll likely know how to deal with it.
The downside to fixing up an old boat is the potential for it turning into a bigger time and and money sink than you imagined. And maybe what starts out as an indulgence in a hobby turns into drudgery. What if money/time runs out and now you've got a half finished boat taking up space?
All things to think about.
Personally, my only experience fixing old boats has been helping my brothers out here and there. One of them seems to spend far more time fixing than sailing. He didn't even get it in the water this year. Earlier this summer a friend of mine asked if I was interested in buying a boat A good friend of his was selling one. I said no. A real boat at this point in my life is probably not in the cards, - for now. I'm in a sailing club and have an inflatable and that will have to do.
Anyway, I figured it would have been a much easier boat to manage for my brother than his current one and it was dirt cheap. I mentioned it to him but he wouldn't hear of it. As big a PIA as his boat has been, he loves that thing.