Get the biggest welder you want to afford and you'll have to tool up from there as well. As I and Ray mentioned, go gas shielded. 1000 times less mess to clean up afterwards (spatter, slag, etc..) plus you can run a lot lower amperage with gas-shielded which is what you want when doing body panels. Metal does crazy (but predictable) things when you heat it with a welder. If you attack a body panel with a booger throwing flux core welder, you will blow through your joints at a minimum and run the risk of overheating the panel and having it warp on you.
Flux core = fast and dirty, good for outdoor welding on windy days with thick steel or if you have nothing else. Not good for body work.
Gas shielded will allow you to use the lowest amperage possible for your thin sheet metal body panels and still get a good weld. You will not be able to run a bead on that body panel with a MIG welder because the metal will overheat and you will blow through. Use multiple tacks, and when that part is done, you can join the tacks with short (no more than 3/8") beads at alternating ends of the panel. TAKE YOUR TIME. Allow the panel to cool between welds so you don't exceed your heat budget.
Here is what I'm talking about with the tacks. A seat I made for a bike I'm building. This is thicker sheet metal than your body panel and I still had to weld like this using my Lincoln 180-amp on it's lowest setting.
When I had it all tacked up, I slowly went back and joined the tacks. Took me a couple hours to make sure I didn't overheat my metal. Once that was done, I went back over the joints with a flapwheel sanding disc (40 grit) attached to my 4.5" die grinder.
Tooling up
If you don't have a 4.5" die grinder, get one. Get some thin metal cutting discs, one thicker grinding disc, a twisted wire wheel, and some flap wheel sanding discs. I recommend DeWalt for the first three (thin, thick and wire) and hit Harbor Freight or eBay for the flap wheel discs because you can get a 10-pack for around $30 which are just as good as the $10/each ones at Lowes/Home Depot.
Skip the chip hammer, you won't ever use it with MIG.
A good pair of mid-forearm length welding gloves are a must and a leather apron is nice. Harbor Freight for these. I weld in sandals most of the time. Safety third. That said, it's a lot easier to kick a piece of slag off my foot than it is to dig it out from between my shoe and my ankle. I weld in shorts a lot too because it's hot in North Carolina most of the time.
Helmet. Auto darkening is key. The $50 Harbor Freight will last for about a year and then you will want to get a better one if you find you weld a lot. Get the Harbor Freight one first.
Hand-held wire brush. Yup, $2 one from HF again.
Surface prep is key with welding. Make sure all paint and rust is wire brushed off the metal. The twisted wire wheel on your die grinder makes this pretty easy. Use the hand held for more delicate stuff. If there is any grease, oil, rubber under coating, paint - anything like that - within about 6" of your welding spot, it's got to come off because it will catch fire. IT WILL CATCH FIRE. You may not be able to see that flame with your helmet on. That flame may be coming from your arm, leg, shirt, etc... It only takes catching yourself on fire a couple times before you raise safety a little higher on your priorities.
Practice, practice, practice and keep asking questions.
