That advice about the threshold is complete nonsense. If you get a nice entry door, it'll come with a rot proof jamb, and the threshold won't rot if installed properly.
If the front of the house gets hit with a lot of sun, don't get a storm door unless you leave the screen in it year round. Otherwise the glass will trap heat, the entry door will get HOT and the finish will eventually be ruined. I have a Larson Signature full length storm door with the glass in it, and it baked the glue around the entry door window to the point where it runs out from the window trim and down the door.
Rectractable screen storm doors are nice, but not if you want to show off a nice entry door. The bar across the middle will ruin the look of the entry door.
Don't get a vinyl storm door, don't get one of the cheapos with a wood core. Get a thick aluminum door with dual closers.