With an indirect water heater, you can go with the smallest boiler possible
to heat your home, but I'd strongly advise against installing a three
section cast iron boiler (of any brand) because the rear fire wall is too
close to the burner nozzle and end cone, causing all kinds of problems.
I don't really understand your original post. First you say, "I had 3
different contractors come up with the same sized boiler in their bids."
Then you say, "All contractors chose a boiler with a range of about
60-110BTU". There's a big difference between 60MBH and 110MBH.
Finishing your basement adds very little heat loss to your home unless the
basement is above ground and has standard windows and doors.

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Yes, I will be using an indirect hot water tank.