i could be wrong, but im quite sure that unlike d2, there isnt anything specific to season play that non-season players cant get their hands on, item wise, legendary or not.
however, there is a season journey which gives you rewards, that can only be obtained when playing seasons. but these are
things like portaits, pets, stash tab etc... yes, the haedrigs gifts will get you set gear but the same set gear can be looted in non-season play.
1. there are no specific ones
2. use the most recent build you can. items do get tweaked over time and the "top" builds are adjusted or re-ordered accordingly
its also important to remember old legenadary items arent updated when patched. so they may be obsolete and require you to find new ones.
sets are updated to the most recent version
3. several sites list viable builds for all classes try this one.
http://www.diablofans.com/basically, you only play seasons if you care about the bling and a forced start over.
bling = stash tab, portaits, pets, penants etc...
also, when asking for help for d3, make sure to state your realm and if you play soft or hardcore.
hope this helps.
This post was edited by C4NTWO on Apr 25 2017 11:10am