Good luck with the welding the only thing I can say is do as much practice as you can before you actually weld your car setting up the feed and the power setting is crucial as you will blow holes or end up with the bird dropping welding that I have seen on repair work that I have had to do again for people over the year.
Make sure you get as much of the sound proofing out as you can as this will set on fire very quickly and so will your car if you are not carefull!!!
Always have a spotter with you with a watering can full of water as I found out to my cost a few years ago when I was welding up a small hole above my windscreen on my calibra, I went in to wash my hand and came out to all the sound proofing on the floor under the peddle box on fire
I only just saved the car from going up in flames on the drive, hot weld spatter gets everywhere and will start fire very easily!!!
If you weld near glass try to cover the glass up I use leather welders aprons or it will burn pin holes in you glass!!
I always replace with 1.6mm steel this is hard to bend and shape but if you can do it it is well worth it as it increases the structural rigidity but normal car body panels are only about 1mm I have found as a rule, And as I have found out again to my cost always wear goggle when grinding I have been up to the Eye hospital to many times in the past to have slivers of metal pulled out of my eye and this is not very pleasant at all until they are fetched out.
And buy the best welder you can afford it is worth it try to stay with established makes, I bought a plasma cutter probably made in china but lets see how long it lasts!!
Regards Martin.
. I'm only just about to buy a welder and teach myself so I can try to fix mine 
