Currently, we are running 5.1.3 Sig 257. I know I am behind and also would like to include any DST updates. If I upgrade to 5.1.5 or 5.1.4, what is the version I will need to upgrade to these Service Packs? Or is 5.1.3 257 enough?
You can go for 5.1.(5) .. minimum requirement for this upgrade is 5.0(1) for CLI and IDM users. This Service Pack includes the S272 Signature Update. Regarding IDS/IPS devices, its always best to be running on latest versions.