I'll mainly be doing some suspension strengthening and fabrication work, but the welder I'm looking at has a min rating of 40A, if I were to want to stitch any bodywork would it destroy it?
Depends why it has a minimum rating of 40 amps. If it's a quality secondhand fabricator welder then it should work well on 1mm plus. If it's a cheaply built new welder it probably won't give any control on the lower settings and could be a nightmare.
BTW that draper of yours looked discounted from the normal discount. But I've never used one so couldn't comment on how nice it is to use.
I think you will find it very very dificult (not imposible but bloody anoying) to do car thickness panels with that. I think mine is 30amp bottom end & thats ok with .6 or .8 mm on typical car stuff (not tried it on the tin foil on a renault though).