So should I just roll for the NPC's "lair" and have him carry anything useful with him, regardless of the individual treasure rules?
There are no guidelines because the past has shown that there are
strongly different preferences among gaming groups, and 5e does not want to lean towards favouring any of them at the cost of the others.
What you should do, is what you want to do!
One important concept that 5e proposes is that of
attunement. This is introduced into the game to set a limit on how many magic items a single PC can use at the same time. As such, it creates a
safety net for you, if you are worried about the PCs getting too many permanent magic items. When you create your own items, it's still up to you to decide which ones need attunement and which not. But with this concept in place, you can feel more free about trying out having NPCs with full magical gear, because attunement can keep the main problem (that of the PCs "inheriting" all the NPCs' magic items after the battle) under check.
If you find attunement is not enough, slap some race/class requirement on top of those items (e.g. orcs using magic items that only work on orcs). You can even have unique individual requirements e.g. "This magic axe was crafted specifically for Thog the Half-Orc Barbarian, and works as a normal axe for everyone else".
Magic items are very rare. An NPC walking around with one is not the expectation, it is the exception. In 5E it is perfectly reasonable for a melee character to go 20 levels and never find a magic weapon (although they'll likely be able to buy a +1 weapon somewhere around levels 5 to 10 ihn most games).
The big problem for you, as a DM, is that if you give an NPC as much magic as a PC would have and the NPC dies, the PCs might get their hands on the items. That can (up to) double their magic items and make them more powerful than expected. Avoid the temptation. If you want them suped up a bit, put a wizard in their service that enchants their arms before battle...
I would not suggest to "avoid the temptation", just to use and abuse the attunements & requirements rules
I hate the no magic item crap that is not D&D I grew up to in the old D&D and then 1e world so I have magic items in my game then again I use custom magic item none of mine has tohit on them. We had magic item just not tons of them back in the old days what made playing fun a nice shinny magic item.
Don't overreact, the 5e system supports such playstyle just as well as the "zero magic items" one.