Definitely go for a Co2 or argoshield mig welder.

Stay well away from clarke or halfords shite though.

practice on old metal until you get rally good and your settings are perfect, then use a wire brush disc on a grinder, clean the area and cut out all the rotten metal, shield the area and you're off.

Get the best welder you can afford though because you get what you pay for at the end of the day.

hope that helps