I can give you some personal insight Nick.
1) get a bigger cylinder than you think you'll need, because we tend to underestimate the welding time required to do a job.
2) get some cans of weld-through primer to protect overlapping or enclosed weld seams from rust.
3) get some anti-spatter spray or paste to condition your gun nozzle - it makes a big difference in the performance of the gun when doing extended welding
jobs.
4) use .024/.025 welding wire with light duty machines. I've tried welding thin gauge steel on cars with thicker wire, and it doesn't work very well when used
with most light duty 120v machines.
5) DON'T forget to reverse the polarity of the machine when running gas and solid wire (i.e. Reverse Polarity -- Electrode Positive) -- I've made that mistake
several times over the years in haste and it's frustrating as heck until you finally figure out you have the polarity setting wrong.
6) the common dual dial flow gauges that come with cheap welders are often of very poor quality and can have a big impact on your weld quality. Better to
get a proper flow meter with the floating ball indicator -- it will make a big difference in the quality of your weld. They cost a bit more, but the hours you save
in frustrating dial adjustments and re-work due to poor welding performance is well worth it.
7) practice making plug welds (aka MIG spot welds) on scrap sheet metal until you can do it without looking, and then run some tests to see if you actually have
good penetration. Many plug welds may look good, but will pop apart quite easily when hit with a chisel. It's quite difficult to get consistent plug welds with
good penetration unless you practice beforehand. It's a deteriorating skill akin to shooting a pistol in that you lose proficiency unless you practice regularly.