Ideally, I would like to use soffit and ridge vents. But, I can't do soffit vents because there are no eaves on this old brick house! Even drip edge vents won't work without major work on the slate roof. Also, because it is a slate roof, the ridge vent solution is not as good as on a composition shingle roof.
BTW, the concrete saw & blade rental is about $100 (not a new blade) and scaffolding will run another $50-100. A concrete contracter will cut two openings (25 feet off the ground)for around $800-900. I still think that I could do it myself but I'll spend the $$$ for a contracter and my wife will be much happier.