ST205 gear is not easy to get so you will be paying top $$$ but it is one of toyota's finest creations. Remember that these engines are close to the 10 year old mark so getting one in good condition can be difficult. You might end up buying a gen 3 motor and needing to do a rebuild on top of that. The ST205 is an exellent base either way.
You can rebuild your ST185 and know that you will have a fresh motor. Run an aftermarket ECU to run your supra 550cc injectors which will be fed by your bosch fuel pump and place a TO4E wheel in your freshly upgraded CT26. Do the basics with a clutch upgrade (recommand 5 puck exedy with sprung center) full exhaust and if money allows a CS front bar with front mount and you will have one sweet package.
It really depends on how much you want to spend and what you want to achieve which you have not told us.