sounds like a good start would probably suggest making your clone veg area a little taller as 2 ft really isnt very tall if you ever wanted to keep a couple of mother plants you will probably need a little more room might increase it to 3 ft. that would still leave you 5 ft to flower in wich is plenty of room for using a 400 for scrog and you would still have room to use other growing methods if you decided not to do scrog.
would probably pick up a decent fan and not bother with computer fans as I dont think they will do the job would suggest 6 inch inline with a speed control so you can turn it down as its a little big for what your need, hook it up to an air cooled reflector on the 400 and exhuasting in the attic.
you will have to monitor your temps and see how they do but most rooms are far from sealed so they will be pulling in some air from around the door but if the temps remain high you may have to make an intake to keep the temps down rather than cutting your door just remove it and replace it with some 1/2 to 3/4 plywood that you can easily cut an intake in and make a disposable door that can be tosssed and the original door put back on if ever needed to.