For the most part, the body appears to be in very good original condition. The only serious spot appears to be the LH rear quarter area.
The accident on the tail end of the quarter maybe the reason your quarter is rusting in other places. Accidents tend
to pull panels around thus breaking the seal and letting moisture in. Hence the floor panel and quarter panel issues.
Need to look at rocker panel as well as this could be plugged or exposed for moisture to get in or be trapped.
It's possible you could have a blockage in rocker and water overflowed into floor panel.
I found no less than 5 rodent nests in my rockers etc. Each nest resulted in a hole that required a patch.

I was able to get a really strong welder for my project. He is an welding inspector and trainer.
He did some pretty awesome work for me. I did some welding myself; engine stand etc. but I left the sheet metal to
the experts. Sheet metal can be very difficult as the metal is very thin.

Based on what I have experienced with my project, I will give some feedback based on how I would do it best practices.
For me the only way is the correct way. Now, grinding every weld and make it smooth and bondo it pretty is left
for you to decide. Basically you want a good solid weld that seals the patch to the body. If you cannot seal
the patch then you have problems.

Here are some observations:

1. Did you get the whole body shell blasted to remove all rust, tar etc.? I tried getting round it but you pretty
much have to do it otherwise you will miss spots to fix. If you have any weak spots the blaster will find them.

2. LH rear quarter. The rear portion of the quarter is beat pretty good already. Going to require a lot of
bondo work time. Bondo is acceptable to do as it won't be that thick but I'm not sure I would put that much $$ into a
rusted panel. Look at the patch he is doing at the bottom wheel well. See the rusted inner wheel well he
will be welding to? The steel has to bend around and attach to wheel well. There are other support panels behind there
he's not seeing. Little chance this is the only issue with this rear quarter. Money better spent on new quarter.
New complete panels will provide you with the best structural bond, rust prevention, lower labor costs and typically will save
you more in the long term.

3. LH Rear floor patch. Patch panel acceptable, but needs to be butt welded. It must be sealed with the weld, don't rely on
seam sealer to seal it.

4. Ask if welder is capable to do butt welds. If he cannot, then don't let him weld your car. Some obvious spots should
be butt welds.

5. IMO, primer I hope is just a quick temp spray and body will be stripped of paint again. Once all paint
etc. is removed you should apply a sealer coat of Epoxy primer/sealer. The first coat of paint is the foundation
of your paint job!

Keep in mind, the shop biggest costs are labor, so they will look for laborious work to bill. Invest
your money in better panels that require less labor and are better steel structure when your done.

Don't know you labor rate but I would expect they will want 30-45hrs to weld, body work to rough primer for just your
rear quarter fixed.

Just my opinions...good luck...look at my pic of my bird below. 7 years in, I'm still working on it.
That's what the car looked like just before I tore it down. Lots of issues can be hidden away for you to find.