It actually might be better to go with a bare bones new one and finance it.
Many new basic trailers/pop ups are 10K or under. My first trailer was like that - a 14 ft Dutchmen I got for 9K with $1000 down. It was traded in on the nice one I have now that was 11K, and my payments have always been $200 a month.
As far as things to get - if you have been tent camping, you probably have everything. If needed, just use your house stuff.
You will need insurance, as well as tags and title. Mine is bundled with car/truck/boat and is around $200 a year.
We have the dealer do preventive maintenance in the fall when they winterize it - usually under $100.