Top is a straight swap, or just take out blank plates and fit glass to exsiting top if you don't have whole thing. Easier to change whole roof though.
As for seats and belts, again yes. All the mounting points are there. Some are hidden under carpet, tape and / or rubber gromets. The only 'problem' you may find is if your doner car is an earlier trak the rear seat might have one central rear clip / mounting point. The later modles had 2. This is not a biggy though. Simply locat the seat to the front mounts then drill holes throught the flour for the rear mount, a coulpe of supper size washers under the floor to spread the strain of the seat, and your good to go.

Any veiws expresed in this thread by me are purely from my own experience, and (sometimes) falible memory. Hope my comments help, but please don't take them as gospel.