I would consider clipless pedals. I don't pull but I like being attached. As for a CO2 gun, I say it depends on how strong you are. I would rather have a frame pump with infinite air than have CO2 and run out. But then again, I am able to pump to around 100 psi which is enough to get me home.
If you plan on riding at night, get lights. I mean real lights! Almost any light will make sure you are seen but you need to see too. Even with them, I hit a rock and ate it pretty hard. So, more the merrier.
I am another vote for a helmet. I personally feel wrong riding without one. But then again, I split my helmet in half on a fall when I was a kid. That would have been my head.
I never go for a ride without some cash, a phone and a bus pass (the busses in my area have racks). I also have digital versions of repair manuals on my phone but hope to never need them!
If you get a computer that doesn't do cadence, skip the wireless. Why add the weight, money and battery. If it does do cadence, then wireless is probably the way to go.