Look to see if there is a 220 hookup, if so get a 220 welder. It will be cheaper electrically to run and provide better welds more quickly. The big names are pretty comparable, you'll find Miller and Lincoln guys same as Ford & Chevy. Personally I prefer a Snap-on MM250 (240V) or MM140 (120V), sometimes you can find deals on all of em on Craigslist. That is where I would find one. There isn't a whole lot to go bad in a good welder. I would steer clear of "suitcase" welders if I intended any heavy/constant use, although there are a couple decent ones. The Clarke 6000 can be found at a decent price for what it is usually.
Plasma cutters are the coolest tool in a mechanics shop. Most can get along without one, but why?
As said, it cannot ever be bright enough.
Loud enough sound system to drown out the obscenities.
Remember to consider the cfm ratings of air compressors. The bigger cfm, the more it can handle. The bigger the tank, the longer in between recharges.
A little about them:
Pancake compressors are small (<3G, .3-2hp) and portable, great for nail guns or off site quick use. Not much else, imo.
Mobile compressors are usually <2.5hp and have a 5-30 gallon tank. These are OK for occasional use, but if used daily, for prolonged periods, or for extreme use (high cfm/duty cycle) they will usually be found lacking.
Most (if not all) of the above are 120V single stage. Here, too, if you have 220 things get neater.
Standup or vertical compressors (yes, some are horizontal) are usually 50+ gallon and 5+hp, 2 stage, and 220V. They make more cfm, hold more air, and cycle less (82 or more db in a garage in winter is VERY loud) - plus can be had for less than 1k$
Or a little more for a little better
Many more models exist, I just use those for examples. HD has Ingersal Rand for 1400$, some other brands a little less. They also make verticle compressors that are single stage, which perform great, just not as efficiently, but they usually cost a bit less. Another good note, if you bolt it (through rubber) to the floor, it becomes much harder to run off with in the night. If you run hard lines for multiple hook in spots or multiple tool use, remember to make everything so that the water will drain down, not get trapped. For instance, invert all the Ts, so the water feeds back to the tank instead of to the tool. Assuming you drain your tank regularly, it will be better for the system.