How much range can I realistically expect?
In good weather and in mixed driving, achieving the 310 miles of range should be no problem. In San Diego and over ~12k miles of driving, I've been achieving 234 Wh/mile vs. the ~235 Wh/mile needed to hit 310 miles. I drive a mix of 60% freeway and 40% local. Once you start heating the cabin, you can expect to see a reduction in range.
What do you think of reliability of the car? Maintenance?
Consumer Reports rates it average. I've only had two types of problems, software and fit/finish issues. The most annoying software issue was phone key reliability, which for me turned out to be an issue with my 3 year old phone. Once I replaced it with a new model, the issues went away. Most of the other minor software issues have been resolved over the past year by software updates. There's always some random, minor software issues, but I find it to be no worse than what I've experienced with infotainment systems on ICE vehicles. I've only been to the service center for fit/finish issues, which I'm guessing won't be as much of an issue for you since you won't be buying a relatively early production 3 like mine. The service center is very good about addressing customer complaints; but they are overloaded, so long waits and poor communication from your service adviser is likely.
Maintenance is simple: tire rotations every ~6000 miles, brake fluid change recommended every ~25k miles, and absolutely required battery coolant change every 50k miles.
Options--I'm a somewhat old guy looking for safety features (especially blind spot monitoring and rear monitoring) and liking the safety package on my wife's new Subaru Crosstrek (highest trim level). How well does the Base level work? Does the Autopilot add a lot? What about the "self-driving" level?
The base level gets you the safety features, like emergency auto brake and such. I find the blind spot warning to be only slightly useful, because the warning is displayed on the center display and not on the mirrors. My other car is an Outback with Eyesight and Tesla Autopilot is generations ahead when it comes to adaptive cruise control smoothness and lane keeping capability. On the freeway, you can pretty much set and forget ACC on the 3, whereas the Subaru system requires constant monitoring for cases that the system can't handle (rapid braking ahead, cars cutting in aggressively, etc.). I also like that there's a lot more granularity in the following distance than the 4-steps on Eyesight, none of which gives me an ideal following distance. On the freeway, AP autosteer can handle all the steering on my commute, although I keep my hands on the wheel at all times (as required by Tesla) just in case. It's nothing like Eyesight, which acts more like lane departure mitigation and causes the car to ping-pong across the lane. If you do a lot of long distance driving on freeways or stop and go driving, then AP is definitely worth it. If you mostly do local city driving, then its value is questionable.