This page https://www.lotro.com/en/helmsdeep indicates that there are 4 parts to Helm's Deep:
The Epic Books (Fight for Rohan's Survival)
The landscape quests (Explore Western Rohan)
The Epic Battles (Enlist in Epic Battles)
The class changes (Revitalize Your Role)
So we know that the Epic Books are always FTP, and the the class changes will affect everyone whether they buy HD or not. <- Free, so not part of the Helm's Deep purchase. This includes the level cap raise to 95, presumably.
And we know that the landscape quests & deeds for a new region always have to be bought. <- Paid with HD purchase.
So, are Epic Battles the feature that makes this an expansion??