If the fabric is less formal, like oxford, chambray or linen, then solids are good for casual or "business casual": light blue (especially ), pink, ecru, white, really any light color. I feel like solid colored poplin/broadcloth works better for true "business attire" shirts. These are a cheaper alternative, if they fit.
After you have several solids, I would prefer checks over stripes (which, again, seem more "business attire" to me).
If they fit you, I think the Black Fleece shirts are even better than the regular Brooks. I think they'll put their spring shirts out in the next few weeks. They're also more expensive
The only downside to linen is that it will wrinkle, but that's also its charm.
For belts, I think Equus Leather (whose owner is a member here) is a good choice for casual belts.