the handling is the best mod you can do, try some eibachs for starters as they are fairly cheap.
decat will give the best results, at mot time you will just have to swap it back. sports cat is for the lazy arse who wants piece of mind (me!). as for exhausts there arnt many that have been properly tested for real gains, if you want a 'nice' exhaust get a blue flame, at the other end of the scale a BTB. its all down to how much £ you intend to throw at your car in the future.