I've been looking over the warlock and he seems to be the most apt to follow the paranormal investigator style. Detect magic, at will, along with a host of sensory powers and UMD/magic item aptitudes make him fit the roll perfectly. Even by focusing in non-combat invocations, he's still got eldritch blast.

Picking warlock invocations is simple enough, but what feats do you think would be good for a character like this (think Constantine)?

Maybe ToM's witch hunter PrC for good measure.