Yikes, sounds like the problems I may have in the future with my "$500" Volvo station wagon. (Going on to $1500 is more like it...)

So far I only have a big hole in the "rocker panel" or Sill (the proper word I think.) under the doors. It's really important to keep the integrity of the 'sills' intact as the frame is quite dependent upon the sill 'beam' holding the floor together. I am just guessing though about your condition. You mean the part of the car under the doors or actually part of the doors??? If your sills are going then most likely your floors are starting to rust too... A moderate tap of a hammer underneigh should tell. If if punches through, It's not sound and needs to be patched or replaced. however 700s maybe a bit different than all the 240s I am seeing... mostly rusting thereabouts.

I was just fixing my under the door hole last night with a belt sander and encountered the 400 dollar fix job a previous owner did about 5 years ago... It was just Bondo!!! And the hole is now fist sized. I was a little depressed but It's still fairly sound with inside and top parts of beam still intact.... But if your whole 'sill' is gone I can't imagine your car having not a lot of rust elsewhere... Need to crawl under there and look for it.

Being not a welder I am in a quandary for saving my car. Believe me the Body Shops do not like fixing the older cars as they know their prices for fixing rust is more than the cars are worth. I've been checking.

If you can't find anyone to fix it, you're on your own. I am perfecting an aluminum (painted coil stock used in home construction) with expoxy fabric backing technique with screws and or rivits to hold it in place on those holes I'm fixing under floor and on "rocker panels" (sills)...

When you can only afford a thousand, plus or minus dollar Volvo you need to know a few things about Do It Yourself.

However if it's a bad as you say get a Body Shop or a welder. And expect about at least 500 bucks for the most minimal repair, 1500 is more likely. Then decide if it's worth it. They usually don't even want to do it for any kind of money.

If you do it yourself at least open up the diseased area and get all the rusty debris and dirt and mud out of there and clean it out and dry it with a heat gun... Then if you only need to drive until fixing time (months if need be...) I patch holes with 4 inch wide aluminum foil faced rolls of window and door sealing tape with bitumius tar backing. (gotten at Home Depot for about 8 bucks a roll.) Spray paint it black and the floor looks like new ... until you poke your fingers into it. Avoid using the spray foam in the lower part of any auto as it seems to collect moisture... (from experience...) You've got to keep the rusted out areas dry until time to fix. I seal the bitumus tape with a heat gun and gloves to press it place if it's not sticking well. Working any patches upside down is tough work.

Be aware any lower body parts (new) need to have an experienced welder to put them in (except for a few fender and door panels.