If i were to sub eso plus today, would I get to do thieve's guild quests and level up the tree when I make a new character, or is it for level cap stuff?
You can go and do the Thieves Guild stuff at any level.But it is good to have your basic skill lines in order so you have the ability to fight well. :}
Any level. Still, it helps to have a bit more then starter levels under your belt... especially when it comes to any possible stealth bonus stuff (duh)
As mentioned prior it is at any level. But if you want to attain the assistant and unlock the final quest in the Thieves Guild you'll have to get your Thieves skill line up to at least level 10.