With mix and match outfits you can get as creative as you want. If you usually wear business suits, buy suits that can coordinate with each other. Also buying some seperates, like a few blouses or a couple of pairs of slacks is a good idea. You can match them with your seperates. Black is a good choice, it always looks good and matches with anything.
Shoes are essential, but also costly.
There are many bargain shoe stores, and all you need are just a few pair to coordinate with your outfits. Scarfs are coming back in style, they look very professional and are very inexpensive. Buy a few with the same colors as the majority of your outfits. Don't over do it and wear them everyday, but a couple times a week is nice. They also look nice when tied around a bun in your hair.