What have you used to start the cycle? Dosing ammonia? ghost feeding? Whatever it is you're using, add a little more (not too much, just enough to read some ammonia in a test). That you're reading nitrite and nitrate is a good sign, but you want to keep that cycle going and build a good population of nitrifying bacteria - otherwise when you start adding fish, ammonia could spike.
You should be fine without one at first with or without the stock lid as long as you're good about manual top offs. And with the stock lid, you'll probably lose even less water. You should have a good ideal already of what evaporation looks like sinee you have water in the tank and it's been running a few days. Has the water level dropped much? I actually drew a line on mine when I first started it to track evaporation.
Personally, I think an ATO is one of the most essential "extras" I own. But I have some sensitive coral which won't do well with parameter swings and we're gone a lot on weekends. So it's a good bit of automation. Some people get by fine with manual topoffs or one of the $15 water bottle ATOs from Amazon.
