If you are certain you will be replacing your current unit-- sell it outright.
Then do not hesitate to "move about the country". That will be one less negotiating point and will also open up access to units for sale by owner who are not interested in taking a trade.
No problem making a deal "sight unseen". Negotiate a price before you leave home-- if the seller knows you drove 1000 miles to see it, he will be a lot less willing to negotiate. Make the offer "CONTINGENT ON SATISFACTORY INSPECTION". Satisfactory is to you-- if you don't like the looks of it, if you don't like the mechanicals, you are off the hook or can renegotiate the price.
I have never had to pay a deposit-- the fact that we left home on the way to see it was good for them-- agreement for 5 days or whatever it takes for you to get there. With dealer, likely a deposit (with the above stipulation clearly on the agreement) should suffice.
We have done this on off shore sail boats, RV's etc.