The recommendation will depend on throughput as well as other factors. The stated throughput for a particular model is unencrypted traffic with no services. If you add encryption, IPS, etc. performance will change.
Take a look at the 5516, 5525 and 5545. These would meet your current bandwidth needs. However, I would recommend thinking about your plans for the future as well as the services that you will be using.
You can compare the throughput, features, etc. of the models side by side here:
https://apps.cisco.com/ccw/cpc/guest/content/ucsSeriesDetails/series_asa5500
Brandon