@W1ngz nailed it.
Over the years you will come to tinker with everything at some point, but for now keeping it simple is the smart way to go. New biomes are going to go through so much change (on their own!) in the first 18 months without too much intervention on your part. Less is more when it comes to tools and additives in the beginning. Take a pass on reactors, dosers, manifolds for now. They are brilliant tools for solving problems that are many months away.
Get a good protein skimmer. The reef octopus 110SSS is so optimal for the 250. Once your tank is fully stocked with fish, it will really help keep your nutrients in order without fuss.
For now, focus your energy and expense on keeping the basics super stable: Temperature, Salinity, and fish nutrition.
Don't rely on a heater thermometer alone. Get a heater controller to accompany your titanium heater. I think the BRS combo is a fine one.
The auto top-off for the 250 is... meh. Do some research on ways that people have installed ATOs on the 250. Simple and reliable (no floods!) is the goal.
As for fish list, I would encourage you to do a lot of research here. It is important to add them in the correct order with an eye on what your ideal stock list looks like. And make sure you understand the ideal diet for each fish. Carnivores, herbivores, omnivores.
Like every forum, there is a ton of great information here along with a ton of... well intended suggestions. Taking note of the the contributor's post history can be helpful.