Alright, somebody who is used to Kayn's Shadow Assassin form answer me this:
What do I build and how should I differ my playstyle from base Kayne and Darkin form?
What do I build and how should I differ my playstyle from base Kayne and Darkin form?
if you go shadow assasin, build him as an assasin, if you go darkin form, go as a fighter with a bit of lifesteal, and maybe a bit of tankyness.
Lethality items are a good choice. Death Dance is an awesome sustain item.
Your W is your main form of damage, don't engage frontally and save your E for escapes unless you need to chase a low HP enemy.
[{quoted}](name=Stars Shaper,realm=EUW,application-id=3ErqAdtq,discussion-id=YpZfP9I6,comment-id=0001,timestamp=2017-07-15T20:03:54.502+0000)
Lethality items are a good choice. Death Dance is an awesome sustain item.
Your W is your main form of damage, don't engage frontally and save your E for escapes unless you need to chase a low HP enemy.
So...

"Item of choice dependent on situation"?
Probably GA would be good
Works too and yes take GA (as fluffytail said) after death's dance (or ebfore if you see the need).
Take 10 CDR in runes, and replace Ionian with sorcs. Free extra damage on passive.
Jungle Kayn, I always start just building my Warrior's enchantment. It fits both playstyles and it does fill that cheap early item slot before you know where you are going with his build. Also, boots are pretty much ubiquitous for both forms. It's more of a "Whatever you need."
If I am laning as Kayn, I normally go ravenous hydra because it fits both roles as well.