I use a 251 every day building at least one chassis a week (2 this week!
) and I love it, but for smaller scale production, I honestly think the 175 is plenty of machine for most everything.
I used to have an Esab 250 amp machine, it has 3 power settings (low, med, high) and 8 smaller adjustments in each power level. I have NEVER taken it out of low when working on tubing, frames, axles, etc. 250 amps is a sh*tload of power, and is really only suited to really thick stuff. Thicker than anything any of our type of vehicles need.
The main benefit of a larger machine for fabricating (imho) is duty cycle. Most single phase welders are only meant to run around 20% duty cycle, which means at full power, 2 minutes of welding in any 10 minute period, to give the machine time to dissapate the heat. Welding less than full power, obviously the duty cycle increases. So if you are trying to weld for hours on end near the max output with a smaller welder, there is a good chance it will overheat and shut off, or worse. For most everyone's garage, that's not a big problem, cause how many hours a week are you really gonna be welding. If its gonna see long periods of use, then you need a bigger one, but it sounds like a 175 would work for your situation.
Oh and definitely get gas, not flux core. C25(Ar75) baby!
Anyways, I gotta get back to work and help Tin Bender SAS and link the front of his yota.
-Redd