Hey there.
I just bought my IS-F today, so I'll answer these from that bias.

It's nearly midnight, and I think I'm going to go take it for a spin ...
Sleep on it. I did, and bought it the next day.
I just bought my IS-F today, so I'll answer these from that bias.
- I have no idea. I'm assuming the car doesn't have Nav/ML? I'm not good at personally modding my stuff unless I'm very familiar with it and have experience.
- cosmetic changes on the interior from what i remember reading. I got an 08, I deemed them not worth the premium for the same thing.
Sounds like a decent price. If that's what they quoted you, there is still probably some more room for negotiating then. Depends how aggressive you want to be and how much you want the car.
It's nearly midnight, and I think I'm going to go take it for a spin ...
Sleep on it. I did, and bought it the next day.