Mike, They are cast resin. They need to be sanded and de-burred but the only parts that need assembly are the wheels and steering column. I have only assembled one, ('66 Mustang convertable) and had a bit of difficulty getting the paint to adhere at first but a quick swipe with 300 grit emery cloth roughed the surface enough for the paint to stick, and not enough to create grooves in the finished project. (solid windshield is a bit of a detraction.)
I find many online in the e-bay n-scale track and accessories section. Don't pay more than 4 bucks a pop. local hobby shops and train shows sell them for 4.99-8.99.