That welder should be more than enough for a go kart, I'm sure you would have no regets buying it. As for the plasma cutter,
1. I haven't really found the need for a plasma cutter all too often (maybe once or twice it would make things easier but they are still possible without one).
2. Not sure of the quality of a NAPA unit. Could be very good, although I feel like it just has NAPA slapped on the side.
That said you may really enjoy having a plasma cutter (I would like one, but not enough to justify the cost), and if that's the case by all means get one!
EDIT: have you at least tried getting a shipping quote from northern tool? They have free shipping on all hobart welders, so maybe a discounted price to Hawaii?