This is what I would do. I can not reasonably claim that this is what you must do...
I would do ALL maintenance (or have it done by an independent mechanic that specializes in Subarus and is someone who won't bullsh!t me). I don't care if the previous owner said he just changed ______. I will do it for my own piece of mind, and more importantly to provide myself a chance to inspect everything I come into contact with. This is not just limited to the engine either.
This will allow me to make a list of things that need immediate attention, things that will need my attention withing the next year, and so on.
Once I have developed a prioritized list of things that need replacing/upgrading (not really keen on "fixing" things if one wants to get into semantics), I can start managing my money and expectations appropriately.
It would help to have more information and some pictures of your car's engine bay and underside if you want more meaningful and specific suggestions.
I suppose if I were to give you a specific suggestion, I would recommend replacing the timing belt, all idler pulleys, and the water pump at the same time. This will allow for a mechanic to get a really good look at what's going on with all of the components he/she touches. On a side note, I was pleasantly surprised at how much easier my engine revved after replacing all of the idler pulleys during my timing belt change. And, I know that it shouldn't need to be done again for quite a while.
One more thing...if your car still has the stock radiator and stock turbo, I would suggest you start saving your money.
The GDB WRX is a great platform, and it will last a long time if you focus on preventive maintenance (re: fix/replace things before they break).