Spend about £50 more and pick up a Clarke TurboMIG 135TE (or very similar) model; much more capable and has provision for shielding gas so you don't have to use 'gasless' wire (which is harder to use, IIRC, and produces less pleasant welds)
Practice on metal of a similar thickness for a while or get someone to show you how to get going and a job like that (hidden behind sill covers) is a good place to start out.
The first 'real' thing I ever welded back together was the fuel tank cradle for a friends Porsche 928 as you can no longer buy the '78 model cradle and he'd have had to update the entire fuel system to the '79 on (at a cost of £500+) to replace it after one of the straps 'fell off' due to rust. It was not a pleasant job but we got there in the end (and it hasn't dropped off again yet..) - I got quite adept at filling holes that I'd blown in the <1mm thick metal
A pair of sills, by contrast, should be a doddle as long as you cut back far enough that you're clear of the crusty metal and into 'good' full thickness steel; just get someone who knows what they're doing to come round when you start (Kevin etc).