welcome to the forum, i seem to have missed the initial posting, glad you stuck to your guns and eventually got one, with regards to the 6 speed i think you'll find it's the cost of the unit over the cost of the fitting, you need the box and rear diff and possibly the prop shaft, the setup second hand seems to be over £1000 easy, weith regards the wind noise, my impreza has wind noise through the drivers window once over 40ish mph, i've not had any water coming in though so it hasn't bothered me, the brakes issue is common on all subarus from what i have seen and the easiest and cheapest fix is a flex stopper, because the metal is so thin to keep the power/weight ratio up the bluk head flexes when you apply the brakes, the flex stopper does exactly that, stops flex, they can be bought new for around £40 and having driven an impreza without one then the same car with one the difference is very noticeable
this is the kit
this is it fitted
there are other makers of it so it's not just the cusco model available