I've used a lot of different ones over the years. miller , Lincoln, Hobart, Esab, Clark, offshore units, etc. They all work. In bodyshops guys will fight over the Miller autosets units while the others sit in the corner. The machine self adjusts so you don't have to climb out and go mess with the wire speed for different welding positions.
Like TechNova said 140a is about at it's limit on a frame rail and why I went with the Miller 211 over the 141. 120/220v , a little more machine etc.
I like infinite heat and wire speed controls, and that would be mandatory on a small machine for me. Get a machine set up for gas. You can always run some flux core until you put some coin together for a bottle.
It's the features you have to compare when shopping. The Lincoln 140 in Home Depot or Lowes is not the same 140 you would find at a welding supply. Miller and Lincoln have some pretty strict guidelines for pricing as a requirement for dealers so you can get the same pricing from a welding supply as internet if you go in with a little info. Dealers on the net can't advertise at less then msrp so there's really no smoking deals to be had buying that way and it's good to get to know the guy that also sells the gas.