I would suggest the 8.0 or 8.2 train unless there is a specific feature in 8.3 that you need to use. The reason for this is that there are very significant changes to the way NAT and ACLs are configured in 8.3, as well as the requirement for a memory upgrade. Here are the release notes for each so you can take a look at features and open caveats in each:
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...