My single pane double hung windows aren't quite 100+ years old (they're about 50 or so), but like you, I didn't want to go through the hugh expense of replacing them with new ones, although many definitely needed attention.
What I did was first take the window sashes out (top and bottom) and using a chisel, knocked out the old hardened glazing compound and removed the little retaining pins (points, I believe their called) then removed the glass. The cross bars (muntins) in mine were generally ok, but had come loose in several places where they go into the rails or styles of the window. I carefully worked them apart, marking where each went (with tape), then hand sanded each one. I only had to make one muntin replacement. I then reassembled. I really debated about what adhesive to use at the junction of the muntin ends: silicone vs. all weather titebond glue vs. construction adhesive vs. polyurethane glue (like Gorilla glue). I ended up using the all weather tite bond. Once reassembled, I primed all of the now bare wood, put the glass panes back in, reinstalled the 'points' and reglazed, The glazing is a little tricky, but you will soon get the knack. I then put on final paint and then reinstalled the sashes.
That was 6 years ago and the 6 windows I redid (on the south side upper floor) still look like new!
For weatherizing, I installed aluminum triple tracks, which have worked well, although I'm just a little bit annoyed at how they look. They don't look bad, its just that their appearance is a bit distracting. Were I to do it again, I'd make some wood framed storm windows: one set using glass, one set using screens, and swap them out each year as the season's change.