An engine build will depend on what you want from it? Just going forged is going to run you about $600-1k if you do it yourself (and buy the couple hand tools). But honestly here's what you should do if you decide to crack open the block for a minimum build and should run you about 2.5k or more: forged pistons, rods, ACL bearings, STi oil pump, STi oil pan/pickup tube/dipstick, and block machining.
You can always do more, but that really depends on what you're building the car for. A street monster (not highway) should be able to scoot around below 4k RPMs, when compared to a highway/track car you'll always be in the RPM range for power (above 4k RPMs). I'd stay away from bigger cams (above a 264) as they shift the powerband above 4k RPMs, vs a 252 cam which would have more umph in the mid range and the upper RPMs just not as much as say a 272 cam. If you go the cam route you should also look into bigger valves and head porting, smaller cams will be more happy with stock valves or a +.5mm, +1mm is going to gain more in the upper RPMs but not as much down low due to flowing too much at lower RPMs. As a rule of thumb, head work will cost you similar to a built bottom end, and isn't really worth it until you build that bottom end anyways because you'll bend/break something before you make the power the heads can flow.
STi rods are usually good for 400hp, I would stay away from Kings bearings as I had a "built" bottom end spin one of their rod bearings, STi oil pump to fight against starvation, STi oil pan/pickup/dipstick also helps keep the oil at the pump pickup (there's a thread about how the wrx pan is known for causing starvation in hard turns, and if you swap pans you'll need the dipstick/tube for that year STi pan or you will not have accurate readings), STi crank as they started making them Nitrate coated and they have more oil flow to fight against starvation at higher RPMs unlike the 205 cranks, KillerB oil pickup for similar reasons but yours has low mileage, they also make an oil baffle/windage tray which helps keep the oil from sloshing up on the crank in hard turns/etc, block machining is a given as over time you create a carbon buildup on the walls and they wear down as well, a quality machine shop should be very knowledgeable with EJs and have a "torque plate" which is required when boring/honing the cylinder wall to keep accuracy due to the nature of the EJ and block flex with that kind of force during cutting. Said shop should also be good with deck clearancing if you use different pistons that require decking.