Twenty yrs ago I've done importing of my own car from Canada to USA (Cali). Also gone other way about 8 yrs ago with a van hauling a 20ft sailboat.
Basically, most of the red tape is centered around new vehicles to prevent or limit cross border shopping, undercutting local dealers/ manfacturers. If vehicle is considered used, things get a whole lot more streamlined. Proof of ownership/title, statement of safety conformity & recalls performed from the dealer manufacturer. I believe you need to send everything to the border some 3-4 days before you get the vehicle there so they are ready to process. Should not be any tax/duty on a used vehicle at border.
Regarding the letter of conformity from a dealer, I actually had a heck of a time getting that from a Mini dealership in California. They told me corporate office was refusing these as import US cars to Canada hurt sales in Canada...no matter if a used car. Had to go to a second dealer and lie that I was trying to sell car on line to an out of state buyer that wanted the letter. This worked. Had absolutely no problems at a VW dealer getting same letter for my van.