Welcome to the hobby!
I started my first tank about 6 weeks ago with the a fluval 13.5 just like you. The light isn't great and it's something you'll probably want to upgrade eventually but there isn't a rush, especially while you're cycling. I'm planning to swap to an AI Blade which can fit in the stock lid with a little modification (this is based off light research but I'm hesitant to guarantee it until I do so).
On an ATO, I've found that with the stock lid you actually have very little evaporation. I highly recommend buying a salt refractometer and the calibration juice for it first, the plastic hygrometers are unreliable and keeping stable salinity is important. The next purchase I'd recommend is an inkbird thermostat. This is a backup for your heater. Usually when heaters fail they fail in the ON position which can cook your tank, the inkbird is a second level of protection from this, just set it 1-2 degrees higher than your thermostat is.
Something else regarding the hobby, overall reef keeping is expensive. It's not uncommon that the tank itself is the cheapest thing you'll buy (there are exceptions of course) You can go slow, watch for deals, get free coral frags and swap with people and save money that way but just be prepared for the nice upgrades to be pretty hefty.
Something I considered with this tank was to buy the Protein Skimmer made for the tank, I'd recommend not doing so. Everyone I've spoken to about it said it did very little for their tank and was more of a headache than it was worth.
If you have any questions feel free to ask me, I'm not the most experienced reefer but I'm more than happy to share the knowledge I've gained so far.
Link to my build thread for this tank.
https://www.reef2reef.com/threads/eastons-fluval-evo-13-5-build.1116814/#post-13723544