I used to install sunroofs in just about anything you can imagine. I did it for a couple years, and even spent a week in Michigan at a Webasto training session.
Popups are simple.. hole in roof... plastic sandwiches the metal with silicone to have a weathertight seal. No big deal.
The full power sunroofs (where the glass drops down and seems to disappear in the headliner) is actually designed to leak. The rubber seal around the glass simply closes the gap. There are four drain tubes located at the corners of inner basket that drain off the water. It leaks water but you never know it. Remember that the inner basket will be twice as long front to back as the sunroof glass and if u add the motor and gearing a little bit longer even.
Remember with any sunroof, the area in which the sunroof will be installed WILL have the cross ribs cut out of the car. On an explorer this isn't support threatening... especially if your going to install a full power model your bracing of the lower basket will support it and be stronger when all is said and done then factory. I pity the guy that puts a popup in a newer Kia or something along those lines and rolls it... those things are like pop cans.
Pricing is as broad as the ocean is wide. You want to stick with a name brand sunroof, Don-mar, Webasto, Leonardo, etc. Pick a shop that'll honor the lifetime installation warranty. Not so much you'll ever need it, because once they are in and working fine thats the way it is. A shop that offers it also takes great pride in their work and will do it right the first time... because its the installer that'll end up redoing it for free, not the shop. You can get a popup installed for just a few hundred bucks, and a full power can hit well over a grand. I used to install Full power roofs in dealership cars, (cars dealerships pick to have stuff put in as a last ditch effort to move the car or because a customer requested a roof put in) for in the neighborhood of 1300 bucks, depending on model of sunroof and model of car.
Hope that answers most of your questions and not confuses you more. Any questions email me off-forum or ask on here!