Hello, Here is a link to the National Highway Transportation Safety Administration, (Federal Government. Reviewing this link may show you what you want in a car seat.
As for a Class B Van at 20K, is not impossible. The point is it will be USED and maybe much more than you want. My suggestion is if you must have a van why not think of a new full-size van with say two rows of seats. NOW ask yourself and your spouse what you will both need and do a conversation. You can also use the van as a run around vehicle, schools, and alike sports activities. Plus it would be fun doing a conversation.
Off the top of my head a place for all four of you to sleep, window blinds/curtains. Port-able potty. In cast into a box, you can use to seat on when no in use. Blader tank for holding water you can move about.
So ask yourself what all could you use a new van?
A conversation starts with some good friends as a fun project. But get your ideas on paper first. Just my thoughts.