Rikandur, you are going to need Epic Leadership to get a cohort who is above ECL 17, and even at your level with your Charisma bonus, at best you would get one ECL 21 cohort with the Epic Leadership feat.
Ah... after re-reading the Possession section in the BoVD, you can't add such a massive bonus via your demon servants.
That's the important part. Thus, you are going to have to choose a single cohort with an ECL no higher than 17. That would be for example:
Succubus (6HD) +6 LA +5 class levels = ECL 17 (11HD) or up to 22,000gp equivalent when she possesses something. Meaning, she couldn't even add the equivalent of a +1 increase on an existing +7 weapon, as bringing it from +7 to +8 requires 30,000gp (98,000 -> 128,000gp).
If you could somehow get a 17HD demon that was ECL 17 (there aren't any), it would still only be 34,000gp. Just over enough to bring your weapon to an effective +8.
Meaning your demon possessing your weapon and adding the equivalent total of +12 in special abilities is right out.
Ah... after re-reading the Possession section in the BoVD, you can't add such a massive bonus via your demon servants.
BoVD said:Enhancer: A fiend can possess a Tiny or larger weapon or armor and enhance it as if it were a magic item. The fiend can duplicate magic item powers worth up to 2,000gp per Hit Die. For example, a Hezrou (9HD) that enhances a longsword could bestow up to 18,000gp worth of powers upon it. It could duplicate the effect of a +3 longsword, or a +1 unholy longsword, or another combination. A fiend that possess magic armor or a magic weapon can increase its power by the same amount.
That's the important part. Thus, you are going to have to choose a single cohort with an ECL no higher than 17. That would be for example:
Succubus (6HD) +6 LA +5 class levels = ECL 17 (11HD) or up to 22,000gp equivalent when she possesses something. Meaning, she couldn't even add the equivalent of a +1 increase on an existing +7 weapon, as bringing it from +7 to +8 requires 30,000gp (98,000 -> 128,000gp).
If you could somehow get a 17HD demon that was ECL 17 (there aren't any), it would still only be 34,000gp. Just over enough to bring your weapon to an effective +8.
Meaning your demon possessing your weapon and adding the equivalent total of +12 in special abilities is right out.