SorryJayME wrote:wow that got wordy :P
I would actually not bother with a wideband o2 sensor. Sure, it will tell you what the mixtures are, but what's the point if you are sticking with the standard ecu? You cannot adjust them, so you might as well tune by knock and spend the wideband sensor money elsewhere. If you change the ecu and tune the car yourself, then definately get the wideband, but otherwise, I'm of the opinion that it shouldn't be above the ecu swap on a limited budget list. I'm far removed from being the most knowledgeable person here, so if i'm wrong, I would be very keen to know why?
T-man, you still haven't specified what your goals are, so all of this advice we're giving you is just us shooting blind.
What do you want? You can stick with the Starion and have a fun car or buy something else and have a fun car. This is personal preference, and again, any advice is blind until you specify what you want.
What's your target power at the wheels? The tbi sohc can manage around the 150-170rwkw mark from most accounts (not necessarily reliable though - this is where mpi comes in, although you might get lucky without it). If you need more, then consider other options - mpi+cam sohc, twin cam, engine swap, or another car.
You've already got an intercooled model, which is a great start! Next step before spending any money at all is setting a concrete goal. "More power" is very subjective and as such will net you a subjective set of answers.