My 03 Impreza rs recently blew its 2.5l sohc engine. My question is, would I be able to direct swap a ej254 Dohc 2.5l engine into my car without having to buy a new wiring harness, exhaust, ecu, etc?

The answer is it depends. If they both use the same sensor pattern then you could make it work ( 6/7 or 36-2-2 ). I'm not sure what the '03 EJ251 uses. The EJ254 has AVCS so you will never get that working and you will need to match power steering lines and other things. If it's coil on plug, then you'll need to adapt plug wires and your coil