Relax! I know it's overwhelming.

Get some platform pedals. I think the W350 comes with the toe cages. Chuck them and pick up a pair of $14 flat pedals and just ride with those and tennis shoes for right now. It took me a year to buy clipless pedals, and probably another 6 months to get comfortable with them. You don't want TWO learning curves going on at once!
A little multitool is a good idea, as is a mini pump or CO2. (I carry both.)

Second water bottle cage and bottles, if you don't have them. Get a bigger seat bag for gels and things, or they can go in jersey pockets.

I'd get the computer, personally. Those apps kill batteries, and I'd hate to be stuck without a phone when I need one.