welcome to the forum,
you,ll need approx 40mtrs of 25x25x1.5 erw steel & depending on if you build the rear tub area as per the book plan or if you decide to use a Triton(grp 1piece tub) rear tub then approx 12mtrs (comes in 6mtr lengths) of 19x19x1.5 erw steel, & also some round tube, tho can,t remember the qty as i didn,t use the round stuff.
all in it should cost you about £130 for the steel to build the main chassis.
as for cutting method............ some used circular saw,s , others use band saw,s or hacksaw,s & cutting discs in a grinder.
good luck with your build
nice birthday prezzie too
oh & the 150 mig you have will do the job no prob.
andy