OP-In a former life I lived in your world. I lived on the Upper East Side.
The most important thing is foundation garments. Get professionally fitted (not at Victoria Secret) by multiple persons bring suits, sweaters, etc Once you have a great foundation you can get a bit more margin of error on clothes and also you can learn your best styles. Most women do not wear the correct bra. You maybe between sizes. This is worth getting correct. A few bras costing $100 can be a good investment (learn how to properly launder). If you need anything readjusted on the bottom get the right shape wear. If you are still young enough that that is not a concern make certain no panty lines, ever! Buy the right fabric, go commando, wear a high cut g, no matter the route you choose no panty lines. Also never underestimate the ability to use TASTEFUL lingerie under a suit to soften the look. Hanro makes some wonderful tops.
Buy multiple pairs of stockings and wash them by hand. If you are between sizes, go up. Don't expect the nylons to make up for any shape wear issues. Yes I know stockings are sort of optional at the moment. Personally I would never attend a business meeting with bare legs (but I am admittedly quite pale, and not in that pretty English Rose way). YMMV
Brooks brothers is ok, but frequently over priced for the "look" and the quality is just ok for the price. I thought their shoes sucked. Also they cut multiple patterns at the same time. This means one suit that is an 8 may have up to a one inch variation on another identical 8. If you buy them often, make certain you try on each item before you leave the store.
For women's suits, please note the list may be out of date:
Lord & Taylor (department store) including their house brand which is both inexpensive and single pattern cuts more in the bespoke tradition. They do this because they have a loyal following and people don't want to have to relearn their size each season.
Nordstroms house brand
VonMaur (midwest) will tailor for free too. They also have free shipping. I made "phone friends" with a sales girl in Iowa and had her call me anytime any high end clothes were 75% off or more in my size. I had a lot of really good solid colored lined wool Pendleton pants (not so popular now) that others were paying $250+ and I picked up for less than $50. I also got tons of camisoles and shirts to go under suits. They used to have much better suiting. I just looked at the site it is lacking.
Donna Karen (only full tailored)
Armani
Burberry-especially accessories (nothing like a cheap plain but well made suit, and a $175 shirt with a hint of nova check)
Escada (looks best on very tall very thin, I have too much torso and not enough leg but I do appreciate the suiting they make)
St. John (sometimes- too much knit and you look like a rich old lady)
Calvin Klein (sometimes they are great other seasons a complete fail)
Ralph Lauren (another hit or miss)
Talbots- basic, classic
The other thing you have to do is think outside the box. A lot of the brands mentioned above have also ventured into the world of plus size. So let's say you are a size 8 and you find a blazer you love that fit's like a glove. You'd like a skirt to match but the store is sold out. Walk on down to the plus size department and look for pants from the same fabric run. You can then take the pants to a good seamstress and likely get 2 skirts made to match. If the blazer was a good deal, it is worth it and nice to have 2 skirts. There isn't much "style" in most lined suit skirts, but having different lengths, slits, or small details to the 2nd make it seem like you have 2 full suits. Also if you bloat, it's nice to have options that week so you could have the skirt tailored more generously.
Another psychological trick of shopping. Every woman will try and remain in the "regular" sized section or petite. Often the first size sold of an item is an XL; however the X that is sitting in the plus sized section will remain and go to deep discount clearance, because women don't want to go to the "dark side". Depending on your cut with some basic alterations these 90% off gems can be a good source of fabric.
Once I had my basic suiting established 8 blue, black, or grey suits. I then moved on to more "interesting" shirts. By changing necklines I could create new looks. You may look at J. Peterman for tops. AVOID the broomstick skirts, but you may find some tops that when paired with a simple skirt will make some nice summer looks.
The other thing I will say is a stylish woman in NYC may not be your best source for help. I got the best most honest fashion advise from gay men. They were more honest to point out flaws, and which trends were a help to my flaws. With women it always seemed more like a competition. I knew if I was in Bloomingdales or Barney's with a man in the woman's department, he wasn't going to lie and tell me something looked poorly on me so he could buy it the next day. You may need to find an older man who remembers when everyone wore a suit to work everyday. Another option is a hair stylist.
I also standardizes somethings in my life. For example I only wear black shoes; many styles all black. This made it easier to invest in the best quality shoes and handbags/briefcase. I only wear real jewelry-less trendy. I have some jewelry that is vintage and others timeless. I know what I can wear with a plain black dress and get a compliment. I also know that when I am done with it the jewelry still has value; don't worry I bought with good discounts. The only other jewelry I wear is made my glass manufacturers: lalique, baccarat, orrefors, kosta boca, daum. These have much lower entry prices for big splashes of color and it gave me a talking point with a billionaire; as if I was wearing art. Since I didn't really have much in common to discuss from my personal life and this added a personal element. After a while clients noticed I wore simple styles of clothes with jewelry and they often gave me gifts. Harry Winston and Van Cleef and Arpels would not be in my drawer if a client had not gifted them to me. Those will also be sold once I am certain my working life is complete (currently a SAHM).
There is also no shame in looking to the media and finding people who's style you'd like to emulate. Diane Keaton only wears black/white. I don't think I have ever seen a picture of her where her clothes did not look great. As I age, I find that type of simplification to be appealing. Quality clothes, just less variations. Your likely much younger than me so she may not be the best example for you, you'll have to find your own.
Also as others have said the cast offs from the high net worth set are a good place to look. If you can't find anything in NYC take LIRR for a ride and see if you can find any good consignment/thrift stores out that direction. The summer wardrobes are often thinned there before returning to the city. Winter wardrobes left in either upstate or in Connecticut. If nothing else LI and CT are the land of cheap mani's and pedi's.
Best of luck