Hmm, the problem with the AI in Skyrim is that it is hardcoded - so without scripts it is quite hard (but not impossible) to change it much. SureAI on the other hand wants to minimize the amount of scripts used in Enderal as the Creation engine can't deal with the amount of detail Enderal has; hence any bigger script mod can make Enderal labil which in turn means many CTDs and the like.
But I'm pretty sure that fights in Enderal will already be harder than they were in Skyrim. Reason 1 being the arcanic fever: whenever you heal yourself with a spell or potion your arcanic fever rises - till you reach 100% and die. the only way to lower it is through Ambrosia, which is really expensive.
The second reason is the static level of your enemies - the world of Enderal will be divided in "level zones". In each of these zones all enemies (other than the boss monsters) have a certain static level.
That means you can actually run into enemies whose level is much higher than yours, hence making the fight that much harder
As for mods that work: basically any texture mod, but once again activate them one by one and carefully test if Enderal becomes labil. Though to be honest SureAI already tries to use textures with as high a resolution as possible without making the game unplayable for those who could barely play Skyrim on medium settings.
As for other mods - Enderal comes with its own masterfile (the .esm file which is first and foremost Skyrim.esm); this means that all Skyrim mods which needed the Skyrim.esm ( nearly all mods which have a .esp file) will NOT function in Enderal and have to be tweaked for using them there.
SkyUI and obviously SKSE are already included in Enderal so you need neither.
As for what mods you should avoid: Well as I said many mods don't work in Enderal in the first place but there still is one kind of mod you should never use in Enderal and these are weather or lightning mods for Skyrim. SureAI got Enderal a new shader and lightning system so any of these mods either look horrible or even cause bugs
If by any chance modders do those for Enderal, too, you are obviously welcome to use them.