heater fans in general are loud.. some of the rear heaters are just blade fans vs blowers which are very loud..
I restored all the factory heaters in both of my busses with OEM style motors and they are quieter.. those rear underseat heaters are crammed under the seats. whereas in your build you can use a quieter aftermarket solution for the rear like eagle did..
im not a fan of aftermarket defrosters but others can choose their own.. when its minus 15 outside I like the main driver /defrost heater to push out 5X the amount of heat those aftermarket ones do..
if you are swappingyour service door for an RV style door then you'll be able to get away with a lot less heat up front..
in one of my busses I kept the right side and center defrost as factory units and restored them.. then for the left windshield unit I installed a Heat / cool unit so I could have true dashboard A/C, refrigerated defrost, and excellent high BTU driver heat...