Maybe do a little reasearch - you cant simply whack a bigger turbo on and wind the boost up for more power...
You need to support the addtional power i.e. Fuel pump, injectors etc etc Also your standard ECU will have a freak out session if its pushed too far.
I would recommend going to speak with your tuner and getting his/her advice. Best to decide on what you want also.
If your only after around 200kw then just get a 2nd hand rb25 turbo for the time being and wait till your ready to upgrade properly - dont half ass it, it will cost you twice as much in the long run.
My 2 cents