phaolo: I think I preferred installing the expansion immediately, instead, since it gave you more items, weapons, stats, etc.
You can still change the difficulty to make the game harder (but beware, it means less exp and no better rewards).
I started with hard and multiclassing and I regretted it lol.
Also, there are mods to export your party to Aranna (the devs' choice to start from zero was nonsensical) or to the multiplayer map for SP (I played those in veteran and elite modes).
Maybe too much travel for such a basic hack & slash game, but I still enjoyed it overall (like a 7\10).
If you followed the guide (I also did at first), then your game also includes this mod:
https://www.nexusmods.com/dungeonsiege1/mods/90?tab=description Which attempts to recreate the original balancing in the Ehb campaign, but there really is no way to tell how precise it is. So the problem is not as bad is it would be without the mod, but the balancing was mostly left to the whims of the mod creator. Given that some of the core mathematical formulas completely changed in LoA, there is pretty much no chance everything is as was intended.
And regarding difficulty, no, it does not affect gained experience in the slightest. Hard just gives enemies +50% damage and you deal 15% less damage. Experience per enemy stays exactly the same because enemies have the same hitpoints and experience values across all difficulties. And you also are actually rewarded for playing on Hard. Weapons on this difficulty sell for 2/3 of their full price, resulting in a very large boost to your gold income. You can also buy an expensive weapon, for say 100K gold and get 66K back when it becomes obsolete. The weapon sale price drops massively on lower difficulties.
This page has pretty much all the info about the game you'll ever need, including many differences between the base game and LoA:
https://dungeonsiege.fandom.com/wiki/Character_Leveling_and_Spell_Guide#Difficulty_Setting And personally, I prefer experiencing the base campaign as it was originally, so without the LoA added items and stuff. Again, the original campaign was not made with LoA items in mind, and they offer a considerable power increase in general.
Considering how easy it is to get LoA to work, I'm actually glad DS 1 comes only as the base game in digital releases.