this topic is debated at least once a month on here and usually ends up with the same opinions being voiced - some by people with valid credentials, and others are just flat out preferences or opinions or mere rehashes of promotional literature. certainly, you aren't going to get full penetration in anything with thick - generally, most people are content to agree that around 1/4" is often obtainable with a good quality 120v welder.
the thing that many people still can't get past is that not everything needs full penetration welds. i have seen 1000's of tons of steel up to and over 1" with 5/16" fillet welds. full penetration is something that is generally avoided unless necessary in building construction. on a smaller scale, look at any farm equipment you have. lots of it is made with intermittent welds, and very little of it would be considered full penetration.
also, make sure to understand the difference between mig and wire feed. many people seem to think that if the welding wire is on a spool, then it is a mig. it is only mig if it uses a gas for shielding. every week i see people on craigslist trying to sell a "mig" welder that only works with flux cored wire - that's not a mig. unless it has (or can be retrofitted with) a gas solenoid for shielding, then it's not a mig welder.
every welder and process has its place. i've used a 120v wire feeder with flux core to weld 1/2" plates to farm machinery for a 3ph attachment, and even after lots of thrashing, everything is still working perfectly and hasn't fallen apart. it is certainly more at home welding 1/8" to 1/4" steel, but you can do limited work on much thicker steel as long as you have the heat to get penetration. large wire feeders with flux core wire are used in construction all the time. multiple inch thick steel is welded with them every day. flux core can be used outdoors, along with stick, while mig is susceptible to gas problems due to wind.
as far as what to get, i personally find the 120v wire feeders with mig capability to be the most versatile welder that a novice can get. you can do very thin up to reasonably thick. it all depends on what you need, if you own a skidder or an excavator, then you're probably barking up the wrong tree, but if you own a compact or smaller tractor, and work with small vehicles (light duty truck and smaller) then the bulk of your work should be within the envelope of a 120v wire feeder.
