You have gotten good advice about cages. Bigger the better! Flight cages, or double flights, are awesome. Get a good variety of natural wood perches of varying sizes.
I'm a teacher, so I'm home all day now for 7 weeks of summer, but during the school year I'm gone for close to ten hours most days. Cockatiels are flock birds. I have two so they can keep each other company. I recommend it. Most tiels are easy going and get along well, but it's not a guarantee. Definitely think about getting two, and if you are thinking of rescue/rehome you might be able to find two that are already used to being together.
I uncover their cage and feed them at about 7 AM as I'm getting ready to leave the house, then when I get home, they get out of the cage time and hang out with me for anywhere from an hour to two hours. I cover their cage and the go to sleep at 7 PM. On the weekends or when I'm home, the cage is open and we play, interact, hangout often throughout the day. They are very used to this routine and don't seem to mind when I'm gone at all. They have each other and lots of toys. Also, there a e people coming and going through out the day; my husband is retired but does a lot of volunteer work, and two of my three kids are in college and still live at home. They talk to the birds, but I'm the only one who takes them out of the cage.
Vet visits vary a lot depending on your location. My avian vet charges $49 for the office visit, full blood work ends up about $200 per bird, but he doesn't recommend that annually if everything is good.
Noise level is not a problem for me. I don't think of cockatiels as particularly loud. Mine welcome everyone back to the flock every time someone comes home, but it's a few quick squawks and not particularly loud. They don't scream just to hear themselves scream. Occasionally a few happy chirps, and Morton will sing periodically, but I find it pleasant and not too loud.
Sounds like you are on the right track for diet. My thoughts are that we actually know very little about avian nutrition, so variety is key. I feed them a little of everything. I buy most of my food through
MSBS. The staple mixture I make consists of
4 lbs Volkman's Cockatiel mix, no sunflower,
1 lbs Caitec Oven Fresh Bites for small birds,
1 lbs Golden Gourmet A Taste of Australia, and
1 lbs Harrisons Lifetime adult superfine pellets. I mix those all up and freeze what doesn't fit in a container I have for the fridge. They get 2 tablespoons of that per day, separated into four different dishes placed throughout the cage.
Every day they get fresh green veggies- broccoli, spinach, chard, kale, brussel sprouts, green beans, beet greens, fennel, whatever we have. They also like shredded carrots. I chop up everything pretty small for them. they like fresh daily- I've tried premaking chop and freezing it, but they don't like it thawed. Along with their veggies they get a spoonful of
Volkman Birdeez Buffet 15 minute Soak and Serve. I make that up in a batch to last about 5 days and keep refrigerated. They also get a spoonful of
Volkman Fancy Soak and Simmer. I make about a half bag of that at a time and freeze in small containers that hold enough for about 5 days and move to the fridge as needed.
In the evening, they get a couple
Nutriberries and
Pelletberries each.
If I see another pellet or food that looks interesting, I'll give that a try, too. My birds are used to a huge variety and will try just about everything.
They also always have
Herb Salad available, cuttle bone, and a
manu mineral block.
My birds like any balsa or natural fiber chewable/shredable toys. There are usually about 6 to 10 in and on their cage. Once a week or so, I'll rotate toys around and take out a few and add a few new ones. Sometimes I'll take one out and put it back in in a week or two, sometimes they destroy them quickly. Favorites for my birds include small toys from
Things For Wings,
I Got A Woody, and
The Best Bird Toys. They also like small
Planet Pleasures toys a lot.
For the bottom of the cage I use plain white paper towels and change every day.
If you can find a rescue cockatiel or two, that would be great. There are so many that need homes.
I waited to respond until I had time to give a very thorough run down, I hope it's useful!
A lot of people think cockatiels are boring because they are relatively inexpensive and very common, but they are really awesome little buddies!
but that's birds for you 
They also LOVE having the radio on, Bazza has been singing along to Triple J all day, and has actually picked up the rhythm of this one really popular song (he sounds absolutely horrible doing it, but if he likes it, I really can't do anything about that). 
