I believe sav_man's 33 N1 which he ran Targa with is the stockest I've seen with just exhaust system done and ecu. Everything else is stock. Pushing more than 300rwkw with more than 95% duty on injectors. So yes it can be done but it all depends on what your objective is. For his, I think he just wanna keep the whole N1 version of the car as much as possible.
Councillor: Do what everyone suggested. My pick for exhaust would be Kakimoto Regu 06&R because of the very distinctive RB brumble they have (yes I've heard a few exhaust and by far, the Kaki's the most distinctive) and the "titanium" looking tip (if I hadnt got my Trust TI-R, I would be getting this). But if you prefer quieter ones, get the HKS Slient. The HKS one will be loud or sound good on high rev but on idle, its pretty docile.
Ultimately if you're not after high power, getting an ecu after the exhaust's been done is required. You will be amazed by how much difference a tune can do. Not so sure about the oil cooler though (its a good to have for street car imo, not a must have). If you need it, get it. If not, dont bother. I wouldnt have a clue because mine came with the factory oil cooler.
I'll say this, DO NOT get dumps for your GTR unless you're doing turbos too. The labour cost is ridicules if you're just doing dumps. And if you're not running really high power, its probably bugga all difference.