By "smaller thrill ride kits", exactly what sets do you have? Are they regular Knex or micro Knex?
But to answer your question, buying a few Screamin Serpents will get you started. The basic minimum is about 2 Screamin Serpent's worth of parts. And smaller than that will basically get you started and then you'll run out. Also, aim for the SS and not the Rippin Rocket to start because the SS has the 3 car train.