Buying caravan/motorhome/trailer interstate can be a bit more challenging than buying one in the same state.
Firstly, make sure you ask the seller all the relevant questions you have about the vehicle and emphasize that you will be travelling interstate and that it is important for them to tell you everything about the item, any damage and any issues before you fly out there. You should understand the possibility of losing any travelling fees if you don't end up buying it.
Secondly, as for payment, Bank Cheque would probably be the most appropriate method of payment as you don't have to carry cash for a long period of time, and the seller can be confident the funds are locked in as opposed to transferring the funds online which can decline.
Need further help? Simply give your local transport authority (VIC ROADS, TMR, Service NSW) a call for more information about registering equipment interstate.