The Bose speakers aren't a big deal to integrate with an aftermarket headunit. There are a couple different companies that make adapters, which simply incorporate a line level adapter so you can dial back the juice. Here is a link to one of the types you can use.
http://www.crutchfield.com/p_1...apter This is what I'm using in my Maxima with Bose with no problems.
For putting in the aftermarket stereo, you can get several different kinds of dash kits... Crutchfield sells some good ones, and you can also luck up on ebay from time to time. From the pictures, it just looks like a standard double din unit. That means the radio brackets off of the stock radio should bolt on and then just drop it in place.
As for the radio removal, I'm not 100% sure, but with most Nissans' and Infinitis', if your pull the AC vents above the radio, and the trim under the radio (usually part of the shifter assembly) then that exposes the screws.
Hope that helps.