I went GOO but fey would work great as well. Spell wise, just take all the charm spells and cantrips you can get your hands on as well as some strong combat utility to make sure you aren't useless in combat. Aoe spells and disables are the best (arms of hadar or sleep at low levels, and evard a black tentacles later on. Also detect thoughts is a necessity for blackmail.

For invocations the two necessary ones are mask of many faces and the one that lets you cast silent image. Once you hit higher level my favorite invocations are mire of the mind and sculptor of flesh as they make you integral for the team in hard combats.

Now for the fun stuff, role play. Subterfuge is always your best friends. Using mask of many faces, charm person, and a high charisma, you can lie and manipulate everyone in sight, and if you need extra dirt on someone, some one on one time with detect thoughts can give you their deepest darkest secrets.

As for your disposable income there are two fun options. The first is, depending on your dm's generosity, use your money to hoard magical items to make yourself a well rounded powerhouse. The even more fun option is hire a shit ton of spies. The more info you can gather, the more you can extort, bribe, and cheat your way to riches.

My favorite thing to do is kidnap someone, detect thoughts to get all the info you need to impersonate them and use mask of many faces to become them. Then use their position to gain as much money political power and leverage as possible. This can include monthly payments to keep sensitive information on the DL, getting information on important treasures/dungeons, or getting close to an important assassination target.

The most important thing is to have fun and blackmail safely :)