I have bags and bags of cars... I think when my nephew hits 4, I'll purge my collection and give him an early x-mas.. My cars of choice are from Jada, Greenlight, M2, and a couple of other brands that scale a bit better than MB/HW. The other thing I would suggest is to also keep them in the same general class, so a Jada Mini will be much bigger than it should be and an F350 smaller, so they look then doubly off. I tend to just not get cars from the smaller end.
I have a small collection started of 1:48 1960's sports coupes like the Porsche Speedster that I plan to gussy up for another game, Machinas, when it debuts. But then I wont get any mid/full sized cars in that set.
Hope this helps!