One hell of a wife ya got there CN!!
I'm all for the simple, cheap, easy way out of things like this. I'd just take a hammerdrill ... or a regular drill with a masonry bit, and just drill a circle 6" around, then go at it with a hammer and chisel. Might take a bit longer than the fancy tool would, but it would save you $$$ and a trip to the store.
Doesn't have to be precise - you can hide all the imperfections with a flue collar & some mortar.