too late Jeff -
macOS 10.13.3 Server no longer works. I have been building Mac servers since 2008, and everything was great until 10.12.6. Now, you can certainly get a Mac (use any Mac that you want - Mac Pro, iMac, Mac Mini) with your Promise R8, as long as you get anything up to 10.12.6 server on it. 10.13.x - forgetaboutit ! No longer works the way you will want it to work.
So what's next ? QNAP, Synology, Lumaforge, Studio Network Solutions, ProMax, etc.
So why did Apple do this ? And when is Apple going to fix this ? OH - I know the answer - because they hate us, and they are never going to fix this. (who knows what will happen with 10.14).