Well it's all a matter of how much money you want ta spend on a new welder. For "most" car /frame building all you need is 130amp and a mig will do,i built my whole frame/cage with my miller 130XP and it did a great job.

If you have a little extra money to spend then a TIG is a very good investment and can really give you some very clean/strong welds.

far as duty cycle that is a consideration but when building a car you really arent continueosly laying a bead for very long so a high duty cycle is not really "needed".

My preference has always been for Miller,my personal welders have all been miller,at work i have a miller 351 & 350LX along with 2 miller migs and have not had a problem with any of them for over 8 years.

I dont know if its still there but i saw a miller 351syncrowave on ebay thursday for $1200 thats a great price and a awsome welder.