Let me dig out my old dyno sheet and see what we are looking at for numbers. IIRC, peak HP was at 5800 rpm, which is probably too high for you. But it also shows a very constant torque curve and if memory serves that peak hp number of a little bump at the end than anything else. I do understand you application and specific needs. I've got a buddy whom I have been helping to develop a Bonneville car. The RPM issue comes up there in the same way because at the salt flats the biggest challenge is getting an engine to hold together until the end of the run at a constant very high rpm that one never sees in a street or any other form of motorsports usage. Your application is not dissimilar other than the fact that you will run it 1500rpm or more lower so that instead of holding together for 5 minutes the engine can run for several hours at a time withouth failure.
Any chance that air cooled builder you are talking to is Raby? If so, his engines are going to run you $10,000-15,000 for the power levels you are talking about.
To answer your question about NA versus turbo Ej25, I personally do NOT support turbo charging the NA Ej25 in stock form. Thicker headgaskets don't solve it because you mess up the timing when you do that. Being a flat 4 configuration you are retarding one side while advancing the other side an equal amount. Personally, I feel that the only correct way to turbocharge them is to add in an STi shortblock or to put STi pistons into the NA engine case.