Let's see...Polar Express mail order $200....Lionel Freight Expansion Set $100 each ($70 mail order or on sale)... two should be about perfect. You will then have 4 freight cars, two operating tracks, two operating cars, 6 more straights, and a few trucks. Lionel sells additional passenger Polar Express cars but they are spendy (about $40 or $50). Passenger cars do look nice on the layout but freight cars have more play value. The K-line cars do work with the Lionel cars but they tend to be a tad smaller (some would say 027 scale).
William's has two lines, Traditional and Golden Memories. I believe the traditional is sized like the current cars in a Lionel set. The Golden Memories might be a tad larger.
Note: If you purchase a set mail order be sure to check that everything works long before the big day! Sometimes sets have a bad transformer.
Jim H