I'm not sure about all of the specific things you are querying, but in general the IPv6 support is much more advanced in 9.x than in any of the 8's, and I would strongly recommend going to 9.0 or 9.1 if you want to run dual-stack with IPv6 enabled. I'm running 9.0 dual-stack currently. You definitely get all of the link layer types, ping, neighbor discovery including DAD, SLAAC, 802.1Q vlan tags, static routes, IPsec tunnels, etc.
Don't forget that the clients will expect to see ICMPv6 router advertisements, and that the firewall (routed mode) or router (transparent mode) RA flags control the client DHCPv6 behaviors if they aren't static. In transparent mode you will have to pass at least ICMPv6 types 133-136 (router & neighbor's solicit &advertise).
You do need more memory to get to 8.3 or later; check the release notes versus your current hardware. The IPv4 NAT is completely different in the later versions (real unmapped addresses in the ACLs, heavy use of the new network objects), also as of 9.0 they unified the IPv4 and IPv6 access lists and groups, so that the "any" keyword is now dual-protocol; there are new "any4" and "any6" keywords for writing single-protocol ACLs.
-- Jim Leinweber, WI State Lab of Hygiene
