If you look at the output of show version from your ASA it will have information about current licensing. If you post that information we could give you better answers.
I believe that every ASA comes with at least some capability to do VPNs. While there might be some possibility of additional license requirements, if this would be the first IPSec VPN that the ASAes are doing then I would doubt that you would need additional licenses.
Several place I have looked indicate that the ASA5520-BUN-K9 supports up to 750 IPSec peers. The SSL is much more tightly limited and likely to require additional licenses. But you indicate that you are planning IPSec VPN so I believe that there is no issue for you.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...