I tried to respond last night but a virus attacked in the middle of it and destroyed my notebook computer completely..... what we endure to be helpful and what some folks won't do to be harmful. (I do have a full-system backup so that is in progress right now. That only makes it recoverable, not acceptable.)
Our 2007 R21-EC does use the flat, 5-pin connector. In our case, we are on an EZ-Loader with surge brakes. Ours are not deactivated electrically, but mechanically, with a couple of interlocks. I find that is not as convenient as electric, especially when I arrive at a launch site and back into a rigging area without remembering to make the changes to the interlocks. The upside is that I can disconnect my electrics before backing into salt water.
Anyway, if the boat you are buying is near any towns with auto parts stores you will be able to buy an adapter easily after you verify what is required. Our adapter came from O'Reilly Auto Parts, but all the majors carry an assortment of adapters. Ours goes from a 7-spade round connector used by most manufacturers with their towing packages to both a 5 and 4 pin flat. It is a Hopkins Towing Solutions #47385 and was about $20.
You might also make a note to ask the seller if he has the connecting wire to the LEDs on the boat if it is not apparent that it is installed. From others posting here, it appears that some trailers have it installed and some do not. It is possible he might have the cable from Ranger but not have it installed.