That trunk floor does look messy. I would not worry so much about rust in those panels and in the wheel wel lip (easily repaired), but I would be far more concerned about more extensive rust in the more structural parts. I would have someone check that out to be sure, before determining how much a reasonable price would be for the car.
I've always believed it is financially best to buy one that a previous owner has fully (and I mean fully and properly) restored, if you intend to keep it for a long time that is.
Good luck with the decision.