What a bleedin' nightmare. Unless you're buying a bare shell, it's always a bit of a worry as to what lurks underneath, especially with these cars as the metal was so thin, nothing lasted too long. Add in accident damage and it just gets worse.
If you think about it logically (not easy), then you're right to go with what's you've got - seeing as you will be changing/amending a lot of the panels anyway. Pleased that that is what is going to happen.
If someone had said, we've blasted it and it's had a new tunnel, a new full floor and sill on one side and a new rear wing, you'd have thought, superb. What's wrong here is that the repair is a) obvious once uncovered and b) badly completed. Which then leads you to think what else is slightly 'out' and has been 'made to fit'. That rear three quarter join below the side window - no words to even say to that. Yes, you've now got to go back and so the work again with fabricated parts (some) but when it is done - and it will be - the satisfaction will be off the scale. Apart from the 'join' and the shocking welding, as for actual rust damage, the rest of the shell is pretty good for one of these.
I've seen cars come back from way worse. It's time and money of course ... but Rome wasn't built etc.
Lots of people on here with knowledge and encouragement - and you've got a good pal in your corner with talent - which is great news.
Plan it all out, get a white board and write it all out in order. Works wonders (or frightens the s&^% out of you .... !)