Having played Torchlight II I can relate.
Unlike that game Diablo characters have no skills and all their stats are capped (see Jarulf's Guide for the starting and maximum stats for each class, there's a thread in this forum with a link to download it). Since eventually every stat will reach its maximum value the notion of a "build" is largely irrelevant, rather you chose how to equip your characters.
I don't regard Diable/Hellfire as particularly difficult though they are challenging. It's a matter of how you chose to approach the game. As nearly as I can gather most jump in with a new character and strike out with the goal of killing Diablo (in a single game). This is very challenging as it requires you be able to play your character skillfully, be lucky with favorable drops and favorable items from Griswold and Wirt (the weapons vendors) while not running into a leader lead group that is especially nasty and places a nearly (or absolutely) impassable barrier between you and reaching the stairs to the next level. But there are other ways to approach the game.
A peculiar aspect of the game is that unlike any later game the vendors, with certain restrictions (price, item level) can sell just about anything that can drop in game. Consequently shopping the vendors, which isn't particulary fun to do, is a time efficient activity. Another issue is that gold takes up inventory space and that limits how much gold you can carry, so there is some incentive to shop the vendors just to keep your gold total under control.
Another feature later games don't have is that you can save the game. If something goes wrong, you die, you touch a cursed shrine, etc, you can reload. Not to mention that at any time of your chosing you can save, start a new game and bring all your equipment (equipped and in inventory) gold, and experience into a new game.
Unlike Torchlight II there is no stash (rather you leave stuff you don't want to carry or sell lying around town. This can be addressed with Ulmo's Stash. See the thread by that that name in this forum.
I regard Hellfire as considerably more interesting than Diablo to play, as it adds some interesting and useful chrome, additional levels, some quite different monsters to deal with, doubling gold capacity and Wirt becomes a much more useful vendor.
Having played this game since the demo first came available two decades ago I could talk endlessly about it. This will have to do for now.