The Mazda is run more regularly during the week, but if it were me, I would definitely go for the Radical! One of the most fun to drive cars in iRacing in my opinion. It's surprisingly catchable in a slide too!
I disagree with the majority of what you've concluded about iRacing and how it compares to other sims in your post. I don't understand how you can say the engine sounds are poor. With the exception of the Mazda, which to me seems like a bit of a bodge job, all of the engine sounds in iRacing are very very accurate, but you do need to turn your volume up once you're ingame. They are also being worked on too, so could improve even more in a near future update!
I'm a fan of the GT series, but this has got to be trolling surely? You can't have given iRacing a serious go if you think rate of throttle applies less than in GT5, especially from the Time Trial demo which has all of 2 variations of the same car. What car(s) from iRacing are you even drawing comparisons from? None of the cars in iRacing are very likely to handle like a tuned or stock 370z.
Some of the things you say about LFS are kind of true, but I still don't think it's fair to say that you can just mash your foot down in LFS and get away with it.
The Lotus is such a fun car to drive. Quite scary at first but so entertaining. My favourite car at the moment by far! So glad I went ahead and bought it.
I got to class A in a matter of weeks! I'm not sure if it is possible to progress from D class in the solstice or legend is it? I thought you needed to race in the class you're at or above to satisfy the MPR needed to be promoted? Correct me if I'm wrong, which is likely, I haven't been in iRacing very long.