Hello Michael,
The ASA 5510 its designed for small offices and medium size companies, has a througtput of 300 mbps.
The ASA 5520 its designed for medium size companies and has a througtput of 450 mbps.
Both of them can have an extra service security module (SSM-CSC,SSM-AIP,SSM-4Gigabitethernet) use to different purposes, this make them a perfect option for the scenario you are looking for.
Hope this helps.
Julio
Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC