As far as brands go, you can break them down into two groups, professional and hobby. The differences are many between the two. Professional machines are quality throughout. The power supplies, contactors, solenoids, drive rollers and motor, ect, ect. The cables and liner are higher quality and don't foul up the wire as often, and they don't fight you in cold weather like a frozen rope. The power supplies are way smoother and supply a more stable arc, which allows you to manipulate the weld pool with gun distance and orientation. The power supplies generally also ramp up instead of a 100% hit on startup. Regardless what welder you buy, purchase a good work clamp. Even $6000 machines come with junk work clamps. Use the stock one to keep your potato chips fresh.
I also always recommend a dual voltage machine. Some day you may have 240v, and 120v is nice too. 120v is great for portability. As stated by others flux is for the farm, gas is for the shop.
For sheet metal a few things will help you out.
1. Butt welding clamps. They not only keep everything aligned, but also maintain your gap.
2. Magnetic copper backing plate. Prevents burn through and works as a heat sink.
3. A Durston plug weld tool for filling holes from trim ect.
4. Panel perfect tool from Eastwood. I've done miles of butt joints without it and this thing is awesome. It saves so much time in post weld dressing.
Welding sheet metal is not running beads, but rather a series of tacks. The key to a good result is good prep and heat control. Take your time and make the pieces CLEAN and fit properly. After you have the machine setup properly it's a matter of distributing the heat and managing panel growth(that's what the gap is for). If you're doing a 12" run, tack both ends and the middle, then keep splitting the difference. 0"-6"-12" are done. Now 3" and 9", then 1.5", 4.5", 7.5", and 10.5". Keep going like that. It distributes the heat and controls panel growth. Also, as stated by others, you want C25 and .023 wire.
Welding helmets: The absolute most important thing you'll buy. Nothing will make you a better welder than a good helmet. There's a huge difference between a cheap auto darkening helmet and a good one.
As far as a specific machine to work on cars, I use a Miller 211. I have big money machines on my field trucks, but the 211 is in my garage at home.