Here is the definitive answer, once again taken from
TornadoAlley's excel spreadsheet for NTSC used card cycles. This is one of
the rarest cars in the used car lots, with only 2 appearances throughout the 100 weeks cycle, making it 2nd only to the Black LeMans cars for rarity value.
Joint 2nd rarest along with Nissan Silvia (CSP311) '65, Renault 5 Turbo '80, Honda S800 '66, Peugeot 205 Turbo 16 '85 and Mazda 110S (L10B).
Here's the 2 day ranges that it appears.
491 - 497 25,269Cr 46,564.7miles
652 - 658 25,269Cr unknown miles (I'm only at day 629 so far)
For those of you unaware of TornadoAlley's sheet, search (using that search function you should all be really familiar with!!!) for posts by TornadoAlley. Find any one of his posts, and there's a link in his signature to the downloadable .xls file. For PAL users, Famine's spreadsheet has its own sticky at the top of the GT4 board.