I keep meaning to get a diesel head for my G4500 too. I just haven't done it yet.
A 24 inch prop size sounds like a pretty good fit for the engine too.
The diesel engines seem to run more quietly than a glow or gasoline engine does. I think it is the way the fuel is burned more completely inside the engine before it is let out. Thus the mufflers can be smaller with less volume to them.

I like your plane choice, it looks pretty good to me.