9mm and .38 Super have close enough rim thicknesses and diameters that they can share the same breech face. A '9mm' slide will work just fine.
I like BARSTO barrels. That's what I used when I converted my first 9mm 1911 to .38 Super.
As for why he's building on in .38 Super, have you tried to locate a .38 Super 1911 recently?
It's never been a real popular caliber and the .40 S&W has overtaken it in the ISPC circles that kept it alive through the '80's. That being said, it can do anything a 9x19mm can do - only faster or with a heavier bullet , at a lower pressure and more accurately. It also seems to shoot 'softer' recoil wise to me.