Safety arguments won't work. Neither will longevity. Nor practicality.

Avoid discussions of logic; this ain't a logical thing for her. Or for you.

What you need is emotional appeal.

How about just saying, "I know it's crazy, darlin', we don't need another old Volvo. But ohh, I really want it..."

Any woman would support that. Because:
(1) It acknowledges her position, which is that buying another 20-yr-old car is nuts; and
(2) eliminates any discussion based on logic (i.e., "convincing," which never works); and
(3) goes to teh heart of it, which is that getting that 240 wagon is a joint decision based on supportive love:

You: I love this car.
She: I want you to be happy.
Both: Let's do this!

P.S. That won't work if $ for car is $ you need for must-do home repair. Or if time that would be spent on 240 is time that's needed for home repair. You know you. And so does she. Don't bother trying to fib.

P.P.S. Got teh phone # for that 240, in case she still says no? We'll give it TLC til after teh divorce.