There are so many rich options for someone in your situation, although region is a factor. If you're in a warm climate, many "country" options may be too heavy.
In your situation, I'd focus first on getting some nice sports shirts. They'll be the most visible thing you wear, you'll need the most of them, and they'll probably be one of the cheapest things you buy. The classic cream-colored tattersall is one of my favorites. If it gets cold in the winter, you can get one in a wool-blend Viyella. They're sometimes available from Lands End and usually from The Andover Shop ($$). I got mine made to measure from Jantzen Tailor and I'm quite happy with it. There are a few big advantages to going this route. First, you'll have a custom fit, which is the best way to look great without outdressing everyone else. The lag on new shirts will also force you to develop your wardrobe at a slower pace, meaning that it will be truer to your style and you will make fewer purchases that don't get much wear. ( Checker #533)
A couple of white and blue OCBDs would also fit in well, especially if you're just starting to build your wardrobe, because they go with everything.
Trousers should be fairly easy, though the trick is to find some that fit really well. Don't ever buy trousers without trying them on, and if you don't absolutely love them, don't buy them. You'll probably be focusing on different shades and weights of chinos. I've actually settled on Target's slim-fit chinos. They're about $25 bucks, they come in summer and winter weight, and several colors. They're slightly low-rise (compared to traditional chinos) but I strongly prefer that look (one thing our generation got right regarding clothes). If you want to mix it up a bit, you can try wool flannels in the winter. I like light grey and loden for such trousers. The more nap they have, the more casual they will look (probably good in your situation).
Now here's the tough part. You'll probably think that my recommendations here are absurd, but I'll plant the seed and let you ruminate on it for a while (maybe a few years):
You should buy a tweed jacket and a pair of cordovan shoes. Expect to pay $1000 for the jacket, and $600 for the shoes. Both items will last a lifetime.
You can topy the soles of the shoes so that you never have to resole them. Read around to learn about cordovan shoes. See some in person. You won't buy them now, but you will eventually. Everyone does. The LHS (penny loafer) is a good option. In your profession, you have a few more options. Personally, I'd go for a cigar shell chukka:
Until you're ready to go for the real deal, these LL Bean Blucher Mocs are a cheap option that would go well with this style.
The fine tweed can wait a little while. You can find some inexpensive options from Orvis or Lands End. Those should work for a few years until you're finally ready to go for something really excellent. This is an important item because it can do everything. You will use it as a light fall jacket to keep warm. Over an open collar OCBD, it won't look so formal as to draw any comments or intimidate anyone. Throw on a tie, and you're ready for almost any event you can imagine these days. The trick is to find a rugged tweed, not a fancy sports jacket that has a faint herringbone homage to the real thing. Natural shoulders will look best. Also, be sure to have the sleeves shortened enough so that you can do things with your hands and not be constantly reminded that you're wearing a coat. The best colors are probably grey, green, or brown. The Andover Shop is a good place for tweeds, but steer clear of the really unique/distinctive weaves. You want something simple if you're going to wear it a lot. My vote for readily available jackets goes to Polo Blue Label. They're made by Corneliani, they have robust fabrics, and the fit is perfect.
For extreme temperatures, you can opt for a polo or a crewneck sweater (always a crew, always over a real dress shirt, with the points always under the collar).