I would strongly recommend a higher tier ASA like ASA 5520/40, they are Gbps interface firewalls for 100 users.
5505 is 100Mbps intarface based, restricted features and no failover capability (well no active/active failover).
For example total traffic is only about 150Mbps via ASA 5505, 5520 gives you 450Mbps, 5540 650Mbps............. 5580 10Gbps
Other considerations are multicontext number, concurrent sessions supported, VPN traffic, expansion, IDS...etc....
See:
http://www.cisco.com/en/US/prod/collate ... 930c5.html