You will need tempered glass if you have a low hung window...If your knee can touch the glass (while standing on the floor) then that window has to be made out of tempered glass. Which will be about an extra $100 a window.
I read online that window brands that boast having insulation in the sill of the window are dumb/silly. You want to have the insulation in between the panes of glass itself in the way of argon (or even krypton) gas and what kind of spacer (You do not want metal!) is used b/w the 2 (or even 3) panes of glass. And you want the low E film too. I think that they called it glazing. And you want at least the double glazed.
We had builders grade 30+ yr old Anderson's and replaced them with Schuco's. The difference has been huge. Now we only shiver (in the winter) if we are in a room with an outside door (we couldn't afford to replace those too)

We also got a wide range price estimates too. We went with a middle of the road price for the best quality product.
What we didn't expect to get was a quiter house. The old windows conducted a lot of outside noise b/c they were made out of glass and aluminum. We paid $8k for 12 windows, 2 with tempered glass. Triple paned/glazed with krypton, half screened, double hung, tilt-ins with no grills, vinyl and white (inside and out).
hth