Sapele will be fine. Every wood has positives and negatives.
On the plus side it's more stable than beech, and a flat bench is a very good thing indeed. On the down side it's bit dark (so for example when you're sighting down the sole of your plane to adjust the blade, you'll need a sheet of paper on a sapele bench to be able to see your adjustment) and sapele's ribbon grain can be a bit awkward to plane.
Regarding width, there's nothing wrong with a 12" bench, but it really helps if there's a something at exactly the same level as the bench top a bit further back (like the rear of a tool tray for example) so that you can rest a larger workpiece level. You'll still only ever do 99% of your work on the first 12" of bench, but being able to support those wider workpieces is useful.