I'm only going to cover a couple point, hopefully someone else will chime in with more info for you.
If you need help with a part, post a picture.
Oil in the plug galley is not unusual when the valve cover gaskets are leaking. That is the most likely source. The other source is the four massive plugs holding down the central cover. Start with getting as much oil out of there as possible with rags and perhaps siphon, then pull the spark plugs and just let the last few drops in each plug recess to empty into the cylinder (I hope there not a lot of dirt in there with the oil). Clean the plugs and wires thoroughly, reinstall the plugs (DON'T OVER TIGHTEN THEM) and run the engine to burn off what drained into the cylinders.
As for the rust, I don't know... The easiest fix IMO would be to cut out what's bad and weld in new sheet... otherwise, if it's more extensive either just cut it all out and weld in a new custom trunk bed or cut one from a chassis at a salvage yard and weld that in.
Check the rear tire wells for rust damage as well.
Your hatch seal is probably leaking if that much water's been accumulating in there.
Yes, cars this old are rarely easy on the wallet.
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