Scale is important to me. I've noticed that many 1:50 trucks are too small, but a few seemed particularly big and worked out well. On the other hand some 1:43 trucks, particularly models of modern Kenworths, etc., are a bit too big.
You did not say what era you model but I do the early mid 50s and there is one inexpensive tractor trailer that works really well for me.
I often search Amazon, using "1:43 truck" to search the toys section. It usually finds 50 or more. New Ray, Schuco, other companies. This one is listed on Amazon for $22 but I've found them for as little as $13.
http://www.amazon.com/1953-Mac...keywords=1%3A43+Mack
I must have bought nearly a dozen of these. I've used some stock, used several of the cabs and part of the trailer to make several two and three-axle trucks, etc. I've converted three to Superstreets.
I have several other brands, too, including a couple of Schuco 1:43 cab over front axle trucks that worked out well, including one Superstreets tractor trailer.- they are actually of more modern vehciles but with a little paint and bodywork I can fake it well enough.