We actually have a very long thread on 1/87 scale (HO scale) vehicles that has been going since 2005 and is still an active thread.
http://www.modelrailroadforums.com/forum/showthread.php?t=2161
The best vehicles to use would be 1/87 scale. The mass producers of diecast miniature cars seldom produce much in REAL HO scale (1/87 scale).
Most Hot Wheels, Matchbox, Johnny Lightning, Greenlight Collectibles, and Racing Champions vehicles and most slot cars, are 1/64 scale, which is "S" scale, and is much larger than HO scale.
In between HO and S scales, is "OO" scale. OO scale was the scale the Matchbox cars and the early Aurora and Atlas slot car sets were back in the late 50's and early 60's. But, since no one in America was familiar with OO scale, they were "marketed" as HO scale, even though they were a bit large for HO at 1/76 scale.
Heres how the scales got confusing:
http://www.hoslotcarracing.com/History.html
As the 60's wore on, slot cars and Matchbox cars grew in size to 1/64, and when Hot Wheels came along in '68, they too were 1/64 scale. But, probably because the slot car track size remained the same during the transition, these were all grouped together as HO scale.
If you use the popular 1/64 scale vehicles that are so easy to find on store shelves, be aware that they will dwarf buildings and people and trains on your layout and make everything around them look smaller than its supposed to look. Bridge overpasses and one story structures will look way too low, vehicles won't be near to fitting in HO scale parking spaces or garages. I measured a properly scaled HO scale car I have on my layout, and this particular model was just over 2" long. I had a Johnny Lightning 1/64 scale diecast of the same make and model car, and it measured just over 3" long over all.
Think of it this way, if you use 1/64 scale vehicles on your layout, a 10' long Smart car becomes 15' long. A 16' long sedan becomes a 24' long limo, and a 50' trailer on a semi truck is closer to being 75' long.
I prefer sticking with manufacturers of HO (1/87 scale) vehicles, such as Classic Metalworks, River Point Station, Ricko, Boley, Brekina, Busch, Athearn, Atlas, Herpa, Noch, Wiking, Road Ragers, NEO, Jordan, Model Power, Woodland Scenics, Trident, and yes, Life-Like too (though Life-Like's HO vehicles aren't very Life-Like in my opinion).
Heres a link to one vendor, Truck Stop Models that specializes in HO scale ( 1/87 scale ) vehicles.
http://www.truckstopmodels.com/
Walthers also has a huge selection of correctly scaled vehicles for HO scale.
http://www.walthers.com/exec/search...ds=restrict&instock=Q&split=300&Submit=Search
There are many other vendors you can try as well such as Buy Hobbies which is an on-line store for Classic Metalworks.
http://store.buy-hobbies.com/index.aspx
Many hobby shops like Burbank House of Hobbies on the west coast
http://www.houseofhobbies.com/
and Model Train Stuff (M.B. Klein) on the east coast
http://www.modeltrainstuff.com/
will also have good selections of HO (1/87) scale vehicles.
A vendor in Europe with a nice selection of 1/87 scale vehicles is Minicars87.
http://www.minicar87.de/
I hope this helps, and wasn't too much information.