More thoughts...
The Imp
So after looking over the Familiar Handbook again, It seems that the familiar is keyed off your *character* level for most of the important things. So lets work with this.
HitDice - Count as equal to your character level. Great, can polymorph it into other nasty outsiders.
Attacks/Saves/HP - Based off your crappy wizard attack/saves/hp, meh...
Skills - If you can get Use Magic Device, that would be great. Your familiar could then UMD with your skill ranks. Maybe buy him a competence ring?
Special Abilities - these are keyed off of your Diabolist level, so natural armor bonus and spell resistance will be lacking.
Decent hide/move silently
Invisibility SLA!
Detect Magic SLA - snag an Artificer's Monocle (MIC) and all your identification needs are taken care of.
Ok, the Imp is pretty cool and flavorful.
However, you can still get this little guy just by taking the Planar Familiar feat (Planar Handbook)
I'm finding it hard to justify suggesting taking the PrC over even standard Wizard (bonus feats), unless the DM is willing to do some very cool things with the "your soul becomes property of a powerful devil".