Check/test the things you mentioned and replace as necessary.

The only thing that ever kills a 240, that makes it financially unfeasible to repair is serious rust. Everything else is cheap enough that with a little work and creativity - and shopping around, talking to the Volvo community, etc. - it will ALWAYS be cheaper to fix a 240 than to buy a new car. And remember that almost no used car in the world is trouble free, and people sell cars after they are tired of repairing things, or after the cars have been neglected so long that they need tons of work. It will ALWAYS be cheaper (in terms of money) to keep a 240 driving than to own almost any other car. You also have to consider the dirt cheap insurance and taxes on a car worth practically nothing.

What is your mileage like? That will be a clue as to how your car is running, and whether you are dumping unburned fuel into the cat.

The AC is a problem; I hear you. I spent several hundred dollars and a LOT of time getting my AC to work last year. It can be done, but ultimately, it is a luxury item, and not 100% necessary in transportation. Even if you spend $1000 dollars getting your AC to work well, it is cheaper than $300/month for a car payment, higher insurance, taxes, etc.

I would say a new car will cost at least $5000/year to own, and a newer used car is going to have the same problems as an old 240, but will probably be more costly and difficult to fix. It will never make financial sense to get rid of your 240. That doesn't mean there are not other reasons to get a different car, but money will never be one of them.