Most of what you are looking for "ability to put together various parts into an assembly, parts of varying materials, check for interferneces, be able to animate and watch them work in unison, be able to analyze stress areas and inherently be able to swap materials to see how they affect those stress points" can be done in the standard package. Exceptions...haha there are always exceptions... you can stress test the individual components, not assemblies in Standard. You can attach "motors" and "springs" in assemblies as well to see how it will work in standard.
If you are looking for full stress analysis, then you may need the full FEA package. Kinematics is only available in Premium, but if your animation needs are simple you may be able to do it in Standard.
I would take your worst case scenario, and ask your reseller to do a demo on a similar part/assembly. They can also show you the yield and deflection you can get with Standard. If you have something in mind maybe the community can look at it and show you.