Pretty crazy how many mods/combos are avail and out there for the Crosstrail. You will love the bike! My lady and I both have Crosstrails and swear by them. We bought the following to get going:
lightweight water bottle cages
waterbottles
A park 3-1 hex tool to make adjustments.
Pedal baskets so I can ride to work in my Vans street shoes.
A Cateye bicycle computer. Admittedly Strava makes this somewhat redundant.
Some great chain lube.

I will say this, the lower end Crosstrails are awesome but fairly heavy. My girl complains to me about the weight of her bike all the time! Be cautious how much money you invest into making the bike lighter. Before you get to far you in will be into "maybe I should buy a Higher end/lighter bike."
Lots of awesome people on this forum with tons of helpful knowledge. Let us know how your build progresses!