It's been too long to really give strong advice. But after having played and GMed 4E a lot, some ideas:
Just use the monster and NPC building rules as guidelines. Do definitely not try to map out the entire spell list of an NPC spellcaster.
Pick maybe 2-4 spells - one he can use as his standard attack (no matter how many spell slots it would require, 1st level spells like Magic Missile or 2nd level spells like Scorcing Ray), one nasty or more aggressive spell (fireball, chain, lightning, meteor swarm, disintegrate, finger of death, as suited by level?) or one nice get-out-of-dodge spell (Dimension Door, Teleport, Mirror Image, maybe even something like Shield or Mage Armor etc.), and maybe another nasty spell that they use when they get into real trouble, or something they can use to buff allies (Heroism, Haste?). They might know more spells than that, but you should know which you want to use in combat beforehand and plan and also improvise around it. And writ them down with the stat block, or have some good bookmarking system.
If you have a prebuild NPCs, still mark the three important spells he will use and leave the rest for off-screen activities, or running a combat with them is too hard.
You might worry that this will mean the CR of the NPC or monster will be off and determing the correct encounter level will be hard, but don't worry: That's true anyway, especially when it comes to spellcasters. It gets worse for monsters like dragons that have special abilities in addition to casting like a spell caster. A Dragon that casts as a Sorceror suddenly gets +4 AC because he casts magic armor is a wild shift in power not accounted for anywhere. And it gets worse with every other buff they might cast! And Dragons are the least balanced monster HD type in the first place, they get good hit points, skill points, attack bonus, and saves and then most Dragons also cast like a sorceror, and their CR is usally lower than their number of HD!
Consider removing cross-class skill cost. Just have cross-class skills limited to half the class skill value. Of course, in the end, the really important skills like Search and Disable Device or Spot and Hide* and what not will still only work well for the people that use them as class skills. But at least it will not feel extra-punsished for trying to leave the straight jacket of their class. Probably also raise the skill points by level for all classes that don't get 8 per level and use INT as main stat by 2, but espeically for those that only get 2 per level by RAW.
*) consider consolidating some skills as well, of course. Climb, Jump Swim go into Athletics, Balance and Escape Artist into Acrobatics, Pick Pocket and Disable Device into Thiefery, Move Silently and Hide into Stealth.
