Hervé said:
I guess you'll have to do it the hard way, especially when most of the foes will be human opponents and not ready to use monsters. Making D20 NPCs is the most boring thing I ever done in 30 years of roleplaying...
Which is why my recommendation is to just not do it. If you insist on playing D20 (and I know you don't, Hervé

), then NPCs have what ever you want/need/think they should have. Don't bother to do all the math and make sure it's "legal". You want them to have a feat or spell? Then they have it. But what is their hit points, base attack, saves, etc? Pick a number that sounds right and go with it.
You don't even have to do it ahead of time. You can figure it out on the fly, as it comes up in your game. Just jot down whatever number you decide on for future consistency, and go. That's what I do. When I look back at my notes for NPCs, they typically have 2 or 3 feats and 3 or 4 other "numbers" (attack roll, damage, AC, etc). Compare 2 feats and 3 other numbers to a fully crafted NPC character sheet for an 8th level NPC. The vast majority of the numbers for the fully crafted NPC never come in play for your typical NPC.
They day I learned the above lesson I cut out half my "work" as a GM. (More than half if I was still playing D20.) I now spend that time thinking up cool character concepts, nasty traps, intriguing plot twists, and so on. Not spending hours creating NPCs who make a two minute appearance before having their heads removed by the PCs.