Welcome. I would refer you to the thread "experience with 21' tow vehicle." The latest posts mention the numbers you need to consider.
Some used 17Bs and 19s just went out the door. If you wait a few weeks, there will probably be one or the other coming along, if there are none left now. Good choice.
Cathy. Floating Cloud
"Live in the sunshine, swim the sea, drink the wild air.... "