One more "big" guy, I'm 6'3", 250 (currently!! hehe), and I've had mine for over a year (6/17/06) and I'm still in love with my car!
I got mine with moonroof and mats as the only "options", a year ago. No leather, no NAV, none of that. I have absolutely no problem with space, though I don't travel in the back so I don't know how much space there is back there. But I didn't try to get this package, I just wanted a basic package, but the first one coming in (within a week) had the moonroof, and I risked waiting another week with a rental if I wanted the same car but no moonroof. I decided to splurge, and I love it (yesterday was a perfect drive with the moonroof wide open!).
Stereo... I'm no audiophile, there are people who complain about subtle inadequacies, but personally I'm very happy with my stereo. No NAV so I have 6 CD in-dash changer, but I've had the same 2 mp3 CDs in my car for probably a year now. I could have over 1500 tracks loaded at once if I wanted!
Seats... I've never had leather so I can't really say about that, but I really like the cloth seats. That might be one to consider your own needs and preferences on--check out the cloth and decide for yourself.
Oh and headroom, I've never had the slightest problem with headroom. I usually drive alone, or occasionally with one passenger (rarely more), so I don't worry about how far back my seat is, but I've got it adjusted so I'm very comfortable. Plenty of room for my legs, my head, comfortable lumbar support....
And MPG, yeah as long as you're prepared to really enjoy the hybrid aspect of your car, I expect you'll do fine.

I never worried about the mileage of my car that much before, but now you have so many ways to track it in this car, and _reasons_ to, that as long as you pay attention to the feedback (instant FE gauge, MFD trip FE graph and tank FE total; NAV I expect has even more) you're going to learn how to take it easy and get a lot of enjoyment out of driving with good mileage--enough to want to keep doing better.
You're not down south though so expect winter months to give you a FE hit. A month or two of cold weather are why I'm down at 36 mpg lifetime--now I'm getting solidly up over 40 (mid-40s even sometimes) consistently, so hopefully I can bring it up a bit... and maybe I've learned enough to help improve my numbers next winter?
But seriously 1 year, and a month and a half with my car, and I'm as excited as ever about how much I love it. Great mileage, very comfortable, good sound, amazingly smooth drive, even using gas it's so smooth and quiet! I hope I can find rental TCHs next time I travel! My last Altima rental annoyed me with how rough and noisy it was--with 2k miles on the odometer!
Definitely try the car out, take your time adjusting the driver's seat to find a position that really makes it work for you.
Then buy it and love it!
ps--oh yeah, and 2007 or 2008? Sounds like 08 is going to be stripped down of a lot of included pluses from the 2007, so if you want them you might save on a 2007. Especially since if you buy soon you can still get the $650 tax credit (if you qualify?) and you _ought_ to be able to negotiate down from MSRP (which I couldn't last summer, it was MSRP or someone else got that car). So there would seem to be some real benefits to picking up an 07 rather than waiting (not to mention enjoying it that much sooner, and using that much less gas!), but what benefits there might be for getting the 08 are much less clear still.