sounds like an easy R&R.

But what condition is the block in that you bought?

And you say an 03 sti? So is it a closed deck 2.0 or a 2.5?

If you are on a budget here is what I would do. Cheap, fast, and fun

Keep you wrx heads, the STI block, shim the oil pump, oem head gaskets, then all the parts from your motor (a/c,ps,ECT...) will just swap over.

With the above give your self a 500budget and you may even have left over if you do the labor, also the hybrids (2.0 heads, 2.5blocks) are seeing over 250 awhp,as where a sti only has 220s.

My brother just built a hybrid for my wrx, and just finished another wrx with just r&r. So if you need help or parts just pm me. I have everything you will need even some tips LOL