I would change the plugs, run it til it dies and get another car after that. I really wouldn’t, but the question I have is: How long do you want to run that thing? Or, better. How much are you trying to save here, and is it worth it to keep the old thing going? It could be worth it to you but maybe not to someone else. I would save most of that money for maintenance after the head gaskets blow out, or use it for a newer car when the big failure happens. You should be operating in worry-free territory here. Your money skills are admirable so I think you will come to the right conclusion.