I need my three players to go capture some guy from his mansion on an island(im thinking). For challenges, at the moment I'm thinking that he has a few flyspy drones circulating for surveillance, 2-3 triad posse as security and he himself is a competent mage.

Is it a bad idea for me to just use the combat mage as him from the rules? Is he particularly bad/good etc?

Other ideas would be good too.

There's nothing wrong with using the Combat Mage as opposition. It's a good time saver, but remember that all of the templates are pretty bland, so you might want to change up the spells, or cyber or bioware. I wouldn't overuse that template, though, or your players will figure things out.

I'd probably up the rest of the opposition and make it more difficult to infoltrate or escape.

It sort of depends on how your players built there characters, and whether or not they have a mage for counterspelling.