The most important fact is missing in your question: How much bandwidth do you want to get through the firewall to the server.
The answer can range from ASA 5505 if 100 MBit/s is enough over ASA 5545-X for Gigabit up to the 5585-X for 10Gig.
And if you want HighAvailability you should at least go for the 5515-X.
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