18.5" is the minimum length unless you want to register it as an SBS. 14-14.5" is common for SBS's. The shorter the barrel, the better due to maneuverability issues. That's for tactical use. For sporting use it depends on what you want to use it for.
For tactical use, a cylinder choke is the most ideal. Cyclinder is actually no choke at all; just a straight bore. If that is not available, the next best option is an Improved choke.