For the first part (tooltip on items to know whether you have them (and how many) in each of your characters):
zz itemsdb:
https://www.wowinterface.com/downloa...z_itemsdb.html - Up to date
BagSync:
https://www.wowinterface.com/downloa...1-BagSync.html(a bit outdated: 8.0 I think)
For the second part, as far as "colectibles" go, the only think I don't get from the default UI (I think) is, whether or not a given token (e.g. Benthic Armor) will give me an appearence I haven't collected yet. For that I use:
TransmogTokens:
https://www.curseforge.com/wow/addons/transmogtokens (a bit outdated, 8.0 I think, but still works fine, for content that didn't come out after its last update).