In AZ, we can tint the rear window and any glass behind the driver/passenger as dark as we want. The driver/passenger windows have to be above a certain shade, but I can't remember what light level that is. From experience, tinting tha back part of a car makes the rest of the interior pretty dark and you really wouldn't want to have your doors as dark anyway...it makes seeing instruments difficult during the day.
As to the windshield, are you referring to having just a visor strip done? The legal method is to run a strip across the top but not below the little mark a few inches from the top known as the "as-1" line. Look for it. I'm 6'2", so I don't even go down that far as it interferes with the view. This is especially true on an off-road vehicle where you may have to look through any part of the glass as the attitude of the vehicle changes as you go over obstacles.
Who in the heck would want to tint their entire windshield?
And you are on the right track having a professional do it. Make sure they are using quality, brand-name film, usually listed as "metallized." Do-it-yourselfers are often VERY sorry they tried to do this themselves.
Don't plan on rolling the windows down for a couple of days to give the film time to dry out/cure, and seek out a non-ammonium-based window cleaner. Putting lots of Windex on a tinted window can make it start to come loose, as that's how they remove old tint.
Your mileage may vary...