The Revell "snap" Peterbilt and Kenworth in 1/25 scale are just the ticket. Both can be found for under $24.00 and the possibilities are endless. Sleepers can be sawed off , frames shortened or lengthened. Lot of guys use the Mobious wheel set to really dress them up.
Just a few that were easy to find
http://www.modelcarsmag.com/forums/index.php?showtopic=94224
http://www.modelcarsmag.com/forums/index.php?showtopic=95301