Doing more research, I'm now beginning to see how broad the subject of kit cars really is. I've since found this Kit Car List
site which covers over 750 different manufacturers
I knew there were more than a few, but I didn't realize there was quite so many choices.
As for the particular models, I'm not really sure, aside from the criteria I mentioned: 2-door, (at least) 2 seats and a heavy emphasis on performance (coupe or possibly a convertible with a good roll bar, but no roadster). Whether or not a kit is for a vehicle that is highly desirable or a replica of a revered classic are not much of a consideration for the characters. It's supercars they're into. The characters aim to one day race in the 24 Hours of Le Mans, so I'm trying to keep that in mind.
One of the two main protagonists is a huge Ford fan, so (after consideration) I'm now fairly certain either a GT40 or GT70 kit would be the character's choice, perhaps with a supercharged Mustang 5-liter V8 engine.
The other protagonist is a Nissan and Porche fan. Since the SL-C was not available in 2005, I was leaning towards the Ultima Can-Am, probably with a Nissan or Porche engine. But I'm looking at other options, too.
I was surprised to see (body) kits for the Carrera GT and I would consider that. However, since the actual cars were manufactured from 2004 to 2006, I really doubt such kits were available in 2005.