Welcome to the forum. Looks like a pretty solid foundation, original paint and a four speed console to boot. With your first quote you have got a good idea what your in for, 5000.00 spread, meaning when he gets into it he would expect to (and probably will) find more under the surface. From what the pictures show you will need some new channels welded in around the rear wind screen. Top of the passengers side quarter may need a patch where the duct tape is. hopefully the rockers and lower quarters and fenders are just surface rust, but patches can be fabbed easy enough for those and welded in. Legendary makes the best repop seat covers, expect to be into the interior for a few grand with someone installing it.
It's possible to do a nice job with the body left on, but the frame will be harder to blast and paint. I would start by power washing/steam cleaning it. It looks like it was undercoated so you may find that you can spot touch-up any areas of bare metal rust with epoxy paint after sandblast to seal it and apply undercoating where they missed from the dealership. Mine looked similar to yours and i found that most of what looked like rust was red clay dust from sitting for 30 years in a carport in Idaho. The way I look at it, if it has protected your frame from rust for 50 years why spend all the time and energy to scrape it off so you can eat off your frame?
You will want to make the suspension and brakes safe before hitting the road. Now for the single most expensive part...body and paint. Not unheard of to get bids in the 8-15,000 range for good body and paint, especially Black as it shows any imperfections in the body work. If you have access to the tools , plenty of time and some shop space you can do it yourself for a couple thousand. I did mine after seeing what others were bid and hearing the horror stories of cars lost in limbo at the body shop. All in all if you can do most of the work yourself your looking at 20K+ to restore one to good driver condition, add another 30-50% if you have to pay someone to do the work. They aren't cheap but they are a hell of a lot of fun.