You will need the head gasket to match the bore of the block and NOT the heads. I'd buy an OE or Felpro gasket set that is designed for your specific block.
Also, you would have no problem running a TD04 if you wanted. And, in order to run the turbo setup you will need a turbo crossmember, WRX/STi ECU and harness, and you will need to have the stock and WRX/STi harness' merged...
You'll need more than that, but those things are the most important things you'll need to take care of.