Most trims (except for the Autech and the Nismos) should have body parts from other versions, including those that aren’t in the game, eg. Series I GTS-25t Type M, or Series II GTS-25t Type MR
With this, here’s my idea on how to make them rare but obtainable at all times:
GTS saloon in the Autoshow, GT-R Autech as its perk reward
GTS-25t Type M coupe in the Autoshow, Nismo 400R as its perk reward
GT-R V-Spec in the Autoshow, GT-R LM Nismo as its perk reward
Which is why I want the game to feature “duplicate” cars. They’d serve different purpouses by having different specs and different upgrades, which really wasn’t the case in GT and still kind of isn’t. Look at the two R32s in GT7, the V-Spec II is cheaper and has a basic widebody option, while the Nismo is much more expensive and has a JGTC R32 replica widebody. The problem is that it was advertised as a new car while barely differing from the V-Spec on surface. Forza shouldn’t go that route and at least include 2-3 variants of the car at launch (R33 GTS, R33 GT-R, R33 LM) and add more as a surprise later. Keep in mind that, again, all of these cars differ in some way and for Forza they’d be much easier to make as they just need to scan one car and then scan other parts, unless they want to finally put effort into it, then reuse the XOne model and scan the other versions that differ in dimensions and number of doors.