The rust through areas you are showing don't cover that large of an area, have you considered media blasting yourself. All you need is a compressor and blasting unit (pressurized ones at Harbor Freight work well). If you are only looking to keep it a driver you don't need to media blast everything, some POR15 will go a long way. If you are doing a nut and bolt frame off, that is where the cost skyrockets.

From the pictures it looks to me like the car was parked outside for a long time and it accumulated leaves, pine needles, dirt, etc. and water settled in and rusted through. Your floor pans and trunk will be dictated by how much the top and trunk seal leaked over the past 45+ years. However, it shouldn't be like the rust buckets we see here in the east and midwest. I envision lots of small patches not large section replacements, but that is my guess. You also may want to start looking for some NOS or rust free units for the front sections (valance and fender extensions).