I'm not following you. The Chuggers with center SS head are $145. The center replacement head, which you could buy and bolt on to the March 815 is $54 which you could do at any point. When you say "base chugger" did you mean the plastic head model? Why waste the money on the $99 pump + new head?
Yes, Chugger heads are 100% interchangeable with March.
On the issue of motor noise, it seems to be completely isolated to each unit. Some are just louder than others. We had a chugger running on the table at NHC and no one even knew it was running. Many people mentioned that their March is louder.