2001 Rover 25 a few issues

i have a 2001 rover 25 1.4i hatchback and ever since i brought it it has had problems. i recently fitted a catback sportex exhaust system to the car and its blowing white smoke out the exhaust and after its been left to idle for a while the idle level drops and sort of flickers a tiny bit and every now and then i get a slight coff its my first car and i realy want to get it sorted and another issue is that it sounds quite tappity but aparntly rover engines do. there is abit of water dripping out the exhaust also but the car is started everyday and not realy driven as i have not yet passed my test i avoid high rev switch offs so there shouldnt be water sittin around in the engine and theres a slight smell coming from the exhaust but aparantly that is normal with a new exhaust i realy hope you can help me understand what these issues might be please get back to me from James