Observers read horizontals first, then verticals. So watch where your jackets end and where the hems on your skirts, pants, and tops hit your body. These horizontal lines should never be placed over the widest part of your hips, legs or arms because they accentuate the figure challenge and make us look shorter and heavier. Make sure those items are hemmed either above or below the figure challenges. You can never go wrong with classics.
Classics are the best investment because they flatter every figure type and they're non-memorable.
You're certainly on the right track with shells, jackets and longer skirts. Cardigan sweaters and vests can work well, too. This third piece communicates more polish and professionalism than just a top and bottom ensemble. A business casual wardrobe is still going to be an investment. You need to look for quality workmanship and fabrics.