First thing I do with a brown dress is hold it up near my face in daylight, not under the shop lighting that flatters everything. Brown splits into two camps fast, the ones that look like good coffee and the ones that look like a cardboard box, and you only find out which camp you're in once you're stood by a window.
If you've got nothing brown in the rail, start with a plain shirt dress in a matte chocolate, not tan, not caramel. It does the same job navy does for a work wardrobe, it reads as considered without trying, and it carries a blazer or a cardigan without a fight. Chocolate works from September through to March, over boots, under a coat, and it survives five days at a desk without looking like fancy dress. Lighter shades, the biscuit and camel end of brown, need proper warmth behind them or they go flat and a bit sad under office strip lighting. I learned that one wearing a dress that looked gorgeous on the bus in and like wet cardboard by mid afternoon in a meeting room with no natural light.
Fit wise, brown shows you up more than black does, every crease and pull sits there in full view, so a size that's slightly too snug across the middle will tell on you by lunchtime in a way the same cut in navy never will. Wrap styles are the usual trap, gape at the bust is worse in a pale brown because there's nothing to hide it, so go up a size and check the tie sits where it should rather than a few inches off centre. Shirt dresses need checking at the hip too, since the brown versions I've tried tend to run narrower there than the black of the exact same cut, no idea why, but it's consistent enough to mention. In the shop, check the lining before anything else, a cheap lining under a wool mix brown dress will cling and ride on tights the second you sit down for a long afternoon of meetings. Check the stitching at the seams as well, brown thread seems to pull loose faster than black on a fair few high street makes, and have a look at the buttons, because the ones that match the dress exactly rather than a shade off are usually the better made version.
The one mistake worth avoiding is buying brown in a shiny or synthetic fabric thinking it will read as rich. It won't, it will look like the colour leaked rather than was chosen, and no amount of flattering lighting fixes that back at your desk on a dull Wednesday.