Boiler BTU Sizing

I have a two family home which is about 800 sqft upstairs 2nd fl(me and wife) and about 700 sqft on 1st fl(tenant). We have two boilers and two water heaters. I did a BTU calculation and came up with about 31,000 BTU's. We have baseboard heaters. We are renovating and putting in new windows and new insulation in the walls. Does this BTU make sense? Based on my web searches it looks like these are $2k plus for a good one.