The application you are looking at is complex. On a side note: Are you building these for sale? If you are, there is a myriad of considerations (ie:Structural design, DOT req., weld integrity, etc.) I own a 115v Lincoln fcaw/gmaw in the 135a range. . . And I love the little guy! I use it with flux-core for portability, wind (excellent for outside), small quicky jobs. . . Though I did fabricate an entire handrail job with it. The capability of the machine to get full penetration in one pass is figured by the EM (not sure how they come up with penetration #'s). If you want to weld 1/2" with it. . . It's possible. It's just a matter of weld preperation, and using multiple passes. Also as you probably know, welding vertical (Up) creates a little better pen. but is harder for a uniform weld. Consider also where your going to work most often? How accessible is 220v, do you want portablility? Before you get fabing on the project, get some scrap 2x4 3/16" and weld on it with the welder you chose. HAPPY WELDING!:)

I own a Hobart 135.... A great little portable unit, but not something I would use for serious fabrication. I don't feel that you get the penetration I like to have in many cases. It is easy to squirt weld on, but you have to pay attention to what is actually happening with the puddle... and of course pay attention to the base metal.
For serious welding I currently use a PowCon 300SM power unit with a Linde feeder (until I blew the feeder up last night....now it runs a home built feeder based on a Chemtron motor and feed rolls, and my own home brewed electronics... works fine). This setup provides enough power that there is no doubt that you are getting a decent weld. Personally I wouldn't settle for the 175 for serious general purpose welding myself.... It just doesn't have quite the punch I like. It isn't that much more than the 135 ..... Remember, these ratings are at very low duty cycles. The 175 may be enough for you.... but it wouldn't satisfy me I'm afraid. The 175 seems to be the price point.... after that they go over $1K. Look at the Hobart machines.... Hobart is now owned by the same company that owns Miller. The small Hobart migs used to weld considerably better than the Miller units, but now they are essentially the same machine, and Hobart has been positioned as the "low budget" brand name. You will find that the units with a multi tap transformer are far less expensive than those with a continuous power knob.... save bucks by getting the multi tap...... continuous adjustment is nice, but not necessary... having enough power to do the job is. Lincoln migs are excellent as are Esab, but both are beyond the Miller / Hobart price range for the same power.

Howard

135 vs 175 it doesn't look like much difference but there is plenty, Note the 175 takes you up to a 220v 20a feed. I've used both sizes (Lincoln)and I like them except for the limited duty cycles. Mig is focused toward production and nothing stinks worse than having to wait on the machine to reset. I've had my 175 for 5 years and really like it. I modified my welding methods to accomodate the duty cycle by spacing out the welding to be done rather than planning on sitting down for an hour at once. The 135 just didn't have the stuff I was looking for.
My advise, spend the $ and jump up to the 175, if your a novice it can be more forgiving on penetration issues. I honestly feel you would be pushing the limit with the 135, given the projects stated.
Randy