bgm1956, welcome to this board and forum. First, know your vehicle's payload (carrying) and towing capacities. This will be an upfront filter for what you can/should be looking at.
Second, the right layout and features/options are very important. Some things you can add or modify but the wrong physical layout will be like a constant stone in your shoe.
Third, be selfish and insist on a used unit that is in excellent or near excellent condition. Don't compromise here as it will likely come back to bite you. Patience is the key and remember, there is always another one to look at around the corner.
Good luck and let us know how this story ends.