i think you may be out of luck, as you have no attic, , where is your current fan, where is it vented to? and where is the little electric box placed (the one thats in the ebay picture beside the fan), the more powerful fans generally are bigger, the main reasons for selv fans are to keep the voltage bit of your fan away from the shower or bath area, so you cannot touch them and get a shock, thats what the little box does, it keeps the mains voltage away from you and the fan and supplys a harmless voltage down to the fan, also you need to get maximum performance from your fan, for instance if your fan sucks 90 litres out of the bathroom through a 4 inch hole, then air has to be sucked INTO the room from somewhere else to replace it, ie 4 inchs worth of a gap somewhere, like under the door or window vent, if it cannot suck in as much as it is sucking out then performance will reduce