well for starters, rapid reload and point blank shot are musts for a crossbow wielder regardless of class.

honestly, for ambush style mage hunting, you'd get better milage out of a rogue than a fighter. If allowed as a playable character a Spellthief might make a good choice. Having to work around that whole double standard of hunting magic users while at the same time being one... Half casting probably wouldn't terribly break a low magic game and since the character would have to constantly worry about becoming the hunted himself, he'd have to be very careful in both spell selection and when/where he casts a spell.

Heck, you could snoop through some of the bloodlines in the SRD for a twist or two