The used lot recycles every 700 days, with the black Le Mans cars appearing in the last week of the cycle (I have 2 of each on my 100% Japanese save). It may be different for the American version I suppose.
I wouldn't worry too much as the Trueno GT-APEX SS comes up reasonably regularly as does the Nissan Bluebird 1800 SSS (in various colours). Not sure about the regualrity of the Skyline. I am sure the frequency of appearances won't vary that much from the Japanese version, maybe the weeks they appear are different. I should have the American version early next week so if no one else has done a check by then I will go though and see what appears..
Famine I could get that Used Car cycle list onto the GT4 section of my Web site if that is any help.