Either i1 Display Pro or Color Munki woudl work well. Very similar colorimeters.
The Color Munki Photo will also profile print media, but the cheaper Color Munki Display can not.
The i1 Display Pro is the only one that can hardware profile the new range of Dell screens though.
It's always been accepted practice to NOT use ambient light control, and instead control your room lighting. If that's not possible, then ambient control can be used, but I found it very inaccurate, and often led to a either a very dim screen in still fairly moderate conditions, or overly bright when in a dark room.
If you're serious about this... and you have the computer in a room where you're not disturbing anyone else, then just buy a black out blind, and a 6500K daylight desk lamp, so your ambient conditions are static. Levels of ambient light are not the only problem... COLOUR of ambient light is the biggest pain in the ass. If you've calibrated your screen to 6500K, then you really should be lighting your room with 6500K. Having a 6500K white point on your screen and normal 3200K room lighting will make your screen appear cold, and subconsciously you'll develop a tendency to create overly warm images.