1 and 2 may be related/the same thing, and 3 & 4 are hard to understand from just your text.
Looking on rockauto, the struts are around $100ea, and it's probably 2-3 hours labor to change a pair on the rear, so that's likely a $500 job. If it's just that, I think it's a no brainer to fix and keep. I'd be hard-pressed to spend over $4000 on the car, as that's about half of what it's worth, but if you really like it and think the rest of the car is in good shape, I'd have no qualms spending that much. (I'm also the guy who keeps my own cars sometimes one repair too long, which isn't disastrous as I do my own work, so the labor is "free", but even with that, you can take it too far.)