ET means offset so how much your wheels stick out towards the fender, The lower the number the further it sticks out and vice versa, I think the max a golf MK5 can take is 8.5 inch wide wheels, offset 45 with 235/35 size tyres, But for that you would also need to do the fender mod, And of course it depends on the offset firstly
Oh yes I have defo seen this r32, you was at the shell petrol pump at whitechapel? Also what brake discs have you got on them brakes?