Not in a Landmark district are you? That can be a game changer.
We usually spec Marvin, or if need be custom, double insulated Low e glass in mahogany sash/frame with bronze zero weatherstripping with profiles matching the original units.
The weatherstripping is as critical as the glass, if not more so- crack leakage is raw pressure differential air flow vs thermal delta thru glass.
Retrofitting weatherstripping to existing decent shape units is very cost effective- and if the window profile is deep enough (IE the windows were high quality when installed) you can retrofit double pane glass...
With interior moldings being important, need to look at units that fit in existing sash space.

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95 E300D gave away
77 300D, 227k, station car
83 300CD 370k, body gone away to the rust gods, engine is in a Yota pickup, going strong
89 190E 2.6- 335k, no more
79 VW FI Bus- 154k summer driver
59 VW Beetle ragtop- 175k
12 VW Jetta- 250k
74 MG Midget-78k