18th level mage's magic items? A set of bracers, two rings, a cloak, pair of boots, a periapt, two wands, a staff and a dagger. What do they do? Oh, does the party want to identify them right here? No? Good, you have time to decide how to allocate the items.
ACs are scaled off of players. I tend towards mages having fairly high ACs due to paranoia, so 23 touch, 31 regular (35 with shield), 18 flat-footed. I can go back and adjust the ratio of magic items/stats/spells retroactively if need be. High level BBEGs are likely to burn XP in stat wishes, which gives me more flexibility in their loot.Very few mages IMCs use permancy, especially as BBEG. Too many dispel magics to waste it.
Sure. Party finds books. Books are warded (possibly just Nystul's aura) and it will take a some time to read through them all and compile a list. I'll get that to the party via email after the game, mmmkay?
I agree that high level casters should not be "spur of the moment encounters" but I have players that will occassionally take a flying leap of logic and decide to do something insane. This means that the "Wizard of Wha", who had been a rumor that I expected to flesh out over the next month, is now starring in the day's session. If they players take a significant tangent, DMs should be prepared to wing it now to make a good game session and then figure out the hows & why-fors afterwards.
Very few players will really take the time to reverse engineer a BBEG to decide if it is legal or not. As long as it played well, have fun.
And if you really didn't like the way the quickly cobbled-together-BBEG folded, have the body dissolve into a puddle of water, ala simulacrum. Poof, you've just indicated he is at least 25% more powerful and the party won't have the same element of surprise next time.