1- the lincon is ok get somthing with gas if you can. flux core is shit. bad penitration and clumply welds = welds breaking and cracking if you are on a buget look for a used one you can get 2 year old ones for like half retail once you have used both there there is no comparison
2-?
3- 16-18 gauge is prefered for floor anything thicker will not form easly for curves ect.
use 14 or thicker for the frame as you want as much strength as your welder will weld
ps the lower the gauge metal the thicker it is
also make shure whatever welder you get has enough amperage to weld the material you are using
for forming metal a crappy hammer and dolly set from princess auto will work to start out. get the peice close tack it in and beat the crap out of it as you weld it down hot metal forms easly
hope that helps