20s are definitely doable although many will tell you its not wise , I am also planning a set of 20s at some point (getting coilovers 1st though) , theres a wheel guide on here somewhere ill take a look in a wee while for you , i know it says with 19s to go no wider than an 8j But if you have a look over in Americana this doesnt seem to apply (they all seem to be riding on coils there though with big rims)
What suspension are you intending to go with 20" rims ?
I cant help on tyre width etc but im sure someone will be along who know better than me .
20s are never going to function in the way your OEM alloys do they will put extra stress on certain parts due to the extra weight but hey i can put up with thta as i think they look immense on the IS.