(You can get the Undermountain Gear [item level ~628 I believe] at Level 70, which seems to be a copy of the Barovia Vistani gear you get starting the Barovia zone.)
So, gear information:
As you start moving through Undermountain, you will start getting green gear, and eventually blue gear, that will have higher item levels then the Undermountain gear. (So, you will start replacing / upgrading piece by piece.)
While I am only just finished with with Twisted Caverns, most of my gear has been replaced once or twice from the Undermountain quest line. In the next three zones, I am sure that will continue!
Weapons are more complicated.
Once you can start running Master Encounters, you have a chance of getting Alabaster weapons, which will be a really nice upgrade.
Also, you can get new seals by running the 'Lair of the Mad Mage' dungeon, and they have end game armor and weapon artifact sets. (Check the seal vendor in Yawning Portal.)
There is also the Watcher Set https://pwimages-a.akamaihd.net/arc/92/4...949008.png
But folks on the forums haven't figured out how it drops yet. ; )