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flux core wire is designed for outside conditions where mig shield gas will be blown away by wind. it also works on thicker material. for welding sheetmetal you need to use .23 or .25 solid wire (70s-6) and a mix gas of argon/co2 (75/25 or c-25 what ever the supplyer calls it). and set your welding machine to the thickness of metal you are welding. on sheet metal you also want to do spot welds as to not warp your metal. if you do a search on the site im sure you will find many topic discussing welding sheet metal.
as far as what machine to get. im partial to the millers but you might be able to get a hobart (made by miller) for a little less. but remember you will have to buy wire and rent a compressed gas cylinder and you will need a welding hood. and that adds money on to what your going to spend on a welding machine