ok, to answer all of your questions:
1) A D15 can be made very powerful, but the only practical way to do it is to turbocharge it. Build the D15 block really tough...forged rods and pistons, cylinder supports (block guard or some other type), balanced crank. You won't have to worry much about the head if you're sticking with a stock redline, but if you want to rev it past that, you'll want stiff valve springs.
2) B-series engine components are not interchangeable with D-series engine components. B-series engines use an 81mm cylinder bore (except for the B20 which is 84mm) and D-series engines use a 75mm cylinder bore. B-series engines are DOHC, while D-series engines are SOHC. I'm assuming you can figure out why SOHC and DOHC cams are not interchangeable.
3) If you want it street drivable, a naturally aspirated D15 will not be competetive against a GSR or TypeR engine in the same car. The only ways to get a N/A D15 to make that much power would be to run an extremely high compression ratio, extremely aggressive cam (wouldn't idle below about 2,000 RPM), and a redline of at least 9k. You'd have a car that would require jet fuel to prevent spark knock, would go through rod bearings every 5,000 miles, and couldn't even dream of passing a smog test.
4) I have not heard of being able to fit a B-series into a EF chassis without custom mounts. If it is possible, I don't think it would be safe. The purpose of custom mounts is to locate the engine in the compartment propely. Maybe it is possible to install a B-series with stock mounts, but it would not be in the proper location, causing center of gravity, weight balance, vertical position and axle position to be incorrect. That would lead to so many problems it's not funny.
5) I would NOT recommend trying to make your own mounts unless you're a skilled welder, skilled fabricator and educated in material properties. If you ARE all of these things, then yes, I honestly do think you could make your own pieces. Otherwise, leave that stuff to the pros. If you were to make your own mounts and one failed, well...just imagine what would happen to you if your car's engine suddenly hit the ground while you were haulin' ass down the freeway.