i unfortunately cannot answer all your questions.
I'm from NY and the costs are insane. labor last i looked was between 100-130$ an hour in many places, i outsourced my car to a shop in CT where at the time the rate was 35$ and hour later the shop went to 55$ an hour.
in most cases you will find much more rust damage then you previously thought. you can tell a shop just what you want and ignore other problems.
there are different methods for welding in new panels, the factory used spot welds, most body shops will butt weld, lap weld, or plug weld. plug welds look like a factory spot weld when dressed up. you want to look at previous work they did and see if you get a reference or not. some places put a new patch over old rusted metal that is bad. they should cut spot welds and replace the entire panel and not cut the rust out and then hammer a patch into the old panel, unless the old panel is worth saving which is rare.
in my case the shop used a spot welder and they went out the the way to try and match factory, over time it has not held up that well and i think know i would of been happier asking them to plug weld. the spot welds were kind of hit or miss in places and the reproduction panels obviously didn't fit like oem so there where finish issues that could not be avoided.
for me enough damage was found that i ended up having to replace the entire front clip. what started as a much more simple job thinking we could get away with patches and some panel replacement turned out to be impossible. the more we peeled back the more problems we found i was in contact with the shop at least 2 times a week since i was 150 miles away.
eventually we both had to come together and decide to stop because it would of just kept going and going. what was suppose to be 3 weeks turned into 5 months, getting the parts was a major problem back then as many reproduction parts today did not exist back then.
now if you want to get into welding a battery tray is a great place to start. I began thinking i would do my own sheetmetal work and i bought a welder, but i just could never find time to practice and start cutting my car apart. in my case it was better i didn't because the work needed far exceeded my patients and skill.
you will want to inspect your frame rails, especially under the upper A arm of the front suspension, rott in that area is death for 71-73 mustangs.
to give you an idea my car the tops of the frame rails on both sides were gone.
you can see the carnage and rebuild here:
http://s1031.photobucket.com/user/72hcode/library/
my adventure cost me about 10,000$ the work was broken into stages with payment breaks and there was a LOT of fabrication that occurred to make parts that didn't exist at that time. plus sourcing and buying parts from all over the place