From what I can tell you would need to buy the full greymoor edition to get all previous expansions. If you get the upgrade you just get greymoor. Summerset and Elsweyr I think can be added from the crownstore but i can't verify. ESO plus gives access to all dlc content (not summerset, Elsweyr or Greymoor) plus a craft bag, 10%xp and gold buff, double bank and housing space.
If you want it for questing I wouldn't buy anything else just yet, base game has tons of lore. (Especially if you want to experience the lore in chronological order)
If for PvP, nothing else needed.
If you want to visit a specific province which is DLC, you might consider buying it.
i also think Morrowind comes with the base game now. But if you want the Full Experience as you said you need the full greymoor edition. The descriptions in the store will tell you what's included. Getting the full version also will give you access to necromancer class, jewelry crafting and antiquities.
All your skill points and "champion points" will probably be reset and need to be distributed again, because they changed a lot for all classes several times. Recent updates have given us free respecs at a shrine for a week, but the first few times they actually reset the skill points for all players. If you left in 2015, you were probably not "VR16" rank, so you when you log in again you will not be "CP160", which is the level cap for gear. Therefore, your gear might need an update after you get some XP and level up to CP160.
A lot of new zones and new gear have been added, and The game has improved in many ways since you left. Cyrodiil is still a mess, but quite a few people are trying to enjoy it anyway.