If you don't want to have the fan and light separate, then you should be able to tie the two wires for the fan and light in the fixture together without any problems.
The vent I put in I bought from Grainger. You can look at their website. They are wholesale, but often don't check walk-ins. Depends on what's on your roof. Assuming it is asphalt, you'll need a special tool - can't remember what they call it. It has a long flat tab with slots. It allows you to get under a shingle, catch a nail with the slots, and it is sturdy enough to use your hammer to pull the nail. If you are lucky, you'll be able to get the shingle out and bend what's left to slide the vent's flange under. The details will depend on what vent you buy. Make sure that either the vent or the fixture has a damper and insulated duct through the attic helps too and will limit the condensation from the cold attic dripping back into the room.