I find this (NSX Prototype LM Road Car) believeable since I have never particularly seen it before. And even I thought I've seen every single Honda in this game...
Not rare at all - start a new PSN ID, fire up gt5, refresh UCD once or twice, it appears immediately. Send it to your main ID and job done.
I started a new ID about 6 weeks ago, this car appeared immeditately -
it also falls under the 1m CR gifting limit, so you should easily get it in the Marketplace no problem.
The RACECAR version is OVER 1m, the roadcar is 500,000 cr.
Forget Mygranturismonet aswell, that only counts what cars people buy, as mentioned, there's alot of pieces of crap no-one (or very few wants), so they aren't in many garages and this artificially increases it's rarity on that site.
All the cars mentioned on this thread are able to be gifted, so they're not rare at all.
The UCD works on a cycle, different cars appear in the cycle in different "rotations". After 1 1/2 years of this game, nothing is rare, there's always usually someone around who can send the car or the ticket to get it, if not, just hit the GT5 website(s) trading area's and youu should pick anything up pretty easy.
Especialy with the recent introduction of all the new lower class tickets too (from Seasonal events), makes things even easier now.
No car is rare anymore, just depends how often you refresh or check the UCD, how often you play the game and how resourceful you are at finding things.