You do not need anything. Not sure what you mean behind xpac. Expansions? UNLESS you want to make a Necromancer character, you need nothing. I guess it all depends also on how much you completed or not completed before.

There is so much in ESO without the expansions you don't need to buy anything. There is no premium membership unless you are talking about ESO+. If you get that, then you get all the DLC with it. You don't even need that if you haven't done much before.

I would recomend ESO+ for one month. This way you can get the craft bag, especially if you are a horder. Also if you do not have the Blade of Woe, I STRONGLY tell people start and do the Dark Brotherhood quest to get it. Once you get the Blade of Woe, you don't need to finish it if you don't want to. Reason why I say get the Blade of Woe is for me, I pretend I am knocking people out instead of killing them (since they come back anyways) and it makes my role play more immersive if I am knocking out people to mug, or guards to get around them.

Other than that, don't invest at all. You might have very bad connection, laggy connection that makes the game no fun.

So first, download and install the game. After all, you bought it so no money needed. Then play. Play before you even get ESO+. If you are a horder and collect all the plants, runes, etc, then stop the game, exit out and get ESO+. If that doesn't bother you then keep playing a bit until you are having fun. Then when having fun, get ESO+.

Unless you plan on spending lots of time in Elsewyr, no use getting it if you are not going to be there. ESO+ has Morrowind. ESO+ DOES NOT GIVE you the Warden though. Not sure if ESO+ gives you Summerset or not.

That is lot of content without Elsweyr. So unless you NEED to start a Necromancer, or want to explore Elsewyr, then it's not needed.

Good luck, hope you have fun and if you have more questions, please ask. If you want me to answer, you will need to quote me since I never know what threads I reply to and never know if someone asks a question or tags me, I may miss it.

Buying elsweyr and getting eso+ will bring you up to speed.