I looked at weather for 2015 and I would have 191 days between 45 and 80. That would be a lot of fun 818c days. The definition of daily driver is a good point. I have pretty low expectations on comfort. I work from home, so many of my outings fall outside stop-go traffic. I don't have to show up at client's offices wearing a suit.

Based on your thoughts, I have formed the following conclusions:

1. This is not a hot weather car. That is OK as making air conditioning work would probably be a pain anyway. Besides, > 80 degrees means I should be riding the BMW or Triumph motorcycles.
2. The 818c does not seem like convertible hard top. Factory Five says that the hard top includes the windshield. To make it a convertible hard top you have to buy the S model body panels. Correct me if I am wrong, but that sounds like more money and a hassle.
3. I should keep the 2004 Honda van around for the days the 818c is not suited. That's OK, it has its place in our world. If there's frost around, its not an 818 day.
4. Solutions like the Vintage air heater/defroster would work for me. Sounds like the car has its own indirect heating. Maybe I just need a sneaky way to clear the fog on the windshield and blast fresh air in the cockpit.
5. Ground clearance is my bigger challenge. My daughter's Scion tC has a 5.2 inch ground clearance and is streetable with caution. The 818 is 4.5 inches at the lowest part of the frame. I definitely need to drive it differently than the van or the Scion. Not sure how easy it is to adjust the height after the car is built. I am aiming toward a fun street car, not a track car. Then again, one track day and I may be hooked into yet another hobby!

Thanks again for all your insights.