Wow I'm surprised by how many think an amateur can repair all that rust. Rust and metal repair is an art.
I beg to differ, there's no "art" in this, it's a skill most people good with their hands can do.
My $0.02: With modern equipment the process of making a single weld spot solid has never been easier. The machine feels the resistance or something and adjust current, and it also sends extra CO2 after button is released to cool down immediately, and other stuff. I don't know all the details, but when I recently replaced my welder, it was a whole new world. The old analogue ESAB had me learn a lot of "moves" to get a weld solid. The new MigaTronic you just enter a setting on material and thickness and it does most for you.
The hard part these days about welding is to me the fabrication of the replacement metal to be welded in. A flat piece is easy, a curved/bent one can take hours to produce. But the good thing is, if you get it wrong, just throw it away and try again, before you weld it in.
Welding requires a lot of patience. It always takes longer than you think. So you can't be in a hurry. But if you have the time, go for it. Also if you source it out, a welder on a fixed charge will not do as good a job as one on hourly pay.
Last, remember what you're welding on. Normally you'd want to do a proper job, obviously, both so rust will never reappear there, also so the needed amount of filler is as little as possible.
How long does it take to repair a rust hole? Depends. A lot! I've welded for many years. Two extremes. I welded a new front footwell in my orig 68 Camaro. I wanted it invisible, both from above and beneath. It took me 2 days, 1½ of those days was used bending the new metal to have the same profiles as orig. A little crazy perhaps as the carpet made these profiles invisible, and not many people looked at the car from beneath. The other extreme was a rusty Jeep Grand Cherokee I got for no money, just to get it legal I did 22 patches in less than 2 days. Pretty? No. Solid and legal? Yes. I still have that Jeep 5 years later, none of the rust I fixed has come back, but new rust has appeared, it's a constant battle
Of course, for a Pantera, you want the best rust repair you can get.
So, there can be many good reasons to let a pro do the job. Lack of time, and too much money. But don't be fooled by "it's an art", no it's a skill, and a pretty simple one at that (just compare to painting a car, now that's complex IMO). And a skill you never regret taking the time to learn. But it's also messy, dirty, a fire hazard.
All that said, with regards to the Pantera looked at here, if it's possible to go over (under) it with a pointy hammer to find all rust, fine, if seller doesn't allow (which is understandable) I wouldn't buy it. Good luck