Fraser, you're going to get a better deal 2nd hand in that price range, IMHO. The weight will be an issue, so I'd be advising that whatever camper you buy should have brakes, or facture the ~$500 to fit them, including a brake controller. It doesn't take much to get to 500kg.
Ideally an alloy camper would suit you, but I doubt there are too many in your price range. You can still get a good camper however.