That doesn't look too bad, but as suggested, there could be a lot more than you think once you start trying to find it - which is something that you need to do regardless of whether or not you want to start finding problems. At the end of the day, it's the main structure of the vehicle, and if you suddenly find a lot of rot, it'd be worth swapping it for a new chassis and you're not far off having it stripped right down there. I suppose you could get away with welding on a patch of metal from a car bonnet, find the thickest one you can and make sure it hasn't rotten through. To be honest though, I'd sooner just go and get some fresh metal from somewhere. Is your friend a big recycler by any chance!? If you do decide to go down the chassis swap route, I don't see the big attraction with galvanised chassis. If you don't paint over it or waxoil it, it'll just start to flake off, and then you're left with a bare chassis. You could get yourself a non-galv'd chassis, prime it, paint it and then waxoil it and it'll last a life time. It all boils down to how much you want to spend, how long you want the job to take and how happy you'll feel driving it once it's done.
-Pos