I think you should go with 17'' or 18'' size for the wheels.
I am looking for ones to myself so I leave you with some sugestions that dont break the bank:
-Kensei KNP
-Rota Grid Drift
-AVID1 AV6 or AV20
Regarding suspension I have in my IS, coils from BC-Racing BR RS they are super fine, but you have a lot of options (Tein, Blitz, K-Sport, etc...)