Screen size doesn't really matter it's the size of the head unit that does. You are looking for a double din. Anyone of them should fit that hole.
Touch screen units have a huge range of prices. The cheap junk ones run around $200 and the big boy no messing around ones can go over $1500. Only you can determine what you need, we can't do that for you. It isn't our money, our car, and some of us might want different "bells and whistles" than you do.
Just put one in my GLi. Was $500 not including the install because I did it myself. That is the price of the unit. Doesn't include any extras like harness, antenna adapter, ipod hook up, nav control, etc.
Keep in mind that if you buy the cheap junk no name ones like on eBay from the shady sellers, you run the risk of them not working right. Just because they say they have all that special stuff doesn't mean that it is compatible with every phone or that they work 100% of the time. I had one for a short period because it was free to me. Will never go with a non-name brand again. The screen was terrible quality, the resolution sucked, the blue tooth worked sporadically, and the screen ended up going out within a couple months. Damn sure was glad I didn't spend any money on it.
