The one tool you HAVE to have to screw around with windows or replace them yourself is a good qualify sawzall or similar high-powered reciprocating saw. That above all else is the tool which preserves sanity in working with old houses. You'll find old windows held in with nails going every way in the world and held with metal straps and to try to get one old window out without the sawzall would take forever; the sawzall just goes right through all that stuff.
Thanks for advice.