The necessity to replace depends entirely on what you are after.

The reality is that you can easily make them much tighter and perform better with new storm windows and weatherstripping/retrofits.

Will that get you the look you are going for...probably not.

If you want to update the look, that is up to you. What material depends entirely on the look you are going for?

What color do you want the exterior to be? Wood, Fiberglass, composite, vinyl, etc can all be color matched so that is not a dis-qualifier for any of them. What you want the interior to look like is probably more decisive in terms of a material selection.

I like the look of wood, and got an estimate to take out the whole window which requires redoing the stucco. The new 3x6 window is $400 plus $100 for dual Low E.... installation cost including stucco is another $600, coming to about $1,000 per window... Does this sound right?
The wood sill is okay, old but not termite damaged.... just the window is single pane and weather stripping is shot. Can't I plop out the old and replace a new wood window? Or replace the glass and have a specialist add the weather stripping??
Also need new French patio doors The estimate for divided lights in wood is $1175, plus $400 for dual low E, and another $600 for installation (no stucco removal on this one..) Does this sound right for wood?
Should I consider another material for Los Angeles near the beach??

THanks!

..."plus $100 for dual Low E"

Unless you are considering selling in the near term or you pay for heat, why would you even consider this?

Do you pay for heat or does the tenant?

tenant pays for heat... I'm considering so they have a comfortable unit and not unnecessarily cold....

Just changing out the windows is usually not going to make that huge a difference in the comfort and certainly not as a result of the glass alone.

What most customers benefit from is the improved air tightness at the window opening and that impact on the feel of the home.