I just bought a Trek 7100 WSD and bought a bike computer (Bontrager Trip 2--has everything except cadence and HR) at the same time, so the shop could install it, and I'm so glad I did. I know I could have gotten a bike computer cheaper elsewhere, but I think it would have been difficult for me to install it well. It attaches with zip ties, and the wires need to be routed just right. It was definitely worth getting them to install it imo. They also installed the kickstand I bought.

I don't have any of the other accessories you mentioned, other than a little underseat bag. I got the Avenir Bigmouth, size small, from amazon for $10, and I like it a lot. It's little and holds everything (essentials) that I need. Very well made with a waterproof bottom. It also comes in med and large. The straps go straight up between the saddle rails, then over and down each side and around the bottom to click together. A third velcro strap goes around the seatpost. Very easy!

Have fun with your bike!