We picked some Plan buildings and vehicles up a yard sale once, and my kids love them! Especially the huge parking garage. They are so pricey, i wouldn't have bought them otherwise, but $20 seemed great for the garage, fire station and service station with about 15 cars/ trucks.
I don't want tracks, since we have so many Thomas ones, and the straights already flip to roadway. I bought a pack of curved Thomas roadway to supplement and called it done- until we were gifted a gently used set of ERTL wooden road track with a McDonalds and a Texaco. I think they like that stuff better, since its more realistic looking with places they recognize. I believe it's been discontinued, though. I think they would like the USPS set that goes with the ERTL set, but can't pay $50 for it!