I have had the 90 amp a/c mig welder for a little over two years. It's actually been a good little welder for what it is. I have made a roof rack for my truck and a cargo basket for a utility trailer with it. I have also made some smaller pieces and have done some minor repair work with it. The best I could do with it, and get a solid weld, was 14 ga steel tubing. Even though it worked very well for that, the machine was almost maxed out. Yes there are mods you can do, and adding a rectifier helped, but it was still limited and they tend to burn out. I didn't feel comfortable welding on my trailer frame with it, so I waited.
Last week I purchased the mig 170. This thing is a beast. Even though I am still running flux core, I get far more penetration. My trailer frame and tongue are 1/8" wall tubing. Fortunately I started with a test piece. My first settings blew a hole clean through the wall, and I wasn't even maxed out. After getting some practice in, I found the right settings and went to work. I have tried everything I can think of, short of causing actual damage, to break or crack the welds. These are solid, and for the first time, my welds actually looked really good.
Yes the 170 amp can use gas, but a bottle and a regulator is another $150 not counting the gas, so I don't have that yet. My advice for those looking to make their own frame, and are considering a harbor freight welder, skip the 90/120 amp and get the 170. It is so much more welder and will handle anything you can throw at it. I even welded two scraps of 1/4" angle just to see if it would. No issues.
Now I just have to wait for the new leaf springs from etrailer and I will have a functioning trailer again, even though I haven't started the camper part yet.