gas mod/con sizing - please help!

I posted this on the main wall before realizing there's a separate forum.. so moderators, if need be, remove the offending post.

Hello everyone, I'm very new to this whole thing so forgive my ignorance.
I recently bought a home in northern NJ and am renovating. part of the renovation is getting rid of old boiler and replacing with something a lot more efficient without a chimney.
So far I got far enough to know that i'd like a mod/con boiler with an indirect water heater tank.
Now i'm researching brands/types/etc.. and came to various discussions of how a mod/con can and should be sized properly... and that following generic calculators can lead to efficiency loss.. so with that said.. here are the specs:
the house is about 3700 sf. Built in 1972.
There will be a total of 7 heating zones. 2 of them will be radiant, the rest will be baseboards. radiant will be in the kitchen ( about 400 sf) and master bath.
House has 5 bedrooms (4 upstairs, 1 down) and 3 total baths

Insulation in most of the house will be new and older parts seem fairly well insulated.
What else do I need to explain to get some idea of how to size it properly?

Thank you all!