Hi Andy,
I'm certainly not an expert, but I have used several types of welders in my repairs over the years.
"Stick" welders are great for thicker steel like chassis members and anything around 2mm plus, but will blow apart thin materials like floor pans etc very easily.
The small gas-less migs are good on thin material but hard to keep a good arc going and easy to end up with a messy weld.
The normal gas mig welders are the way to go with the thin floor materails but i'd check out hire prices for a weekend rather than splurging to buy one, if you can get all the steel prepared first (the hard part) should be able to weld it all in place over one weekend.
When you are buying your steel i'd get zincalume, this has a layer of protection over the steel for areas that are concealed, but make sure you grind any steel areas you need to weld.
If you havn't already you may want to check the parts suppliers, a lot of rust repair sections are readily available for areas that commonly rust.
Good luck! :(
Dazza