Step one is to insulate those concrete walls. This lessens the cost of
heating as well as the amount of output needed from the boiler.
The proper way is to calculate the heating needs for the house. Only that
will determine the actual need. However, you are adding some heat to the
basement, not adding more square footage to the house. You also state the
new boiler is 20k more Btu than the old one. I'd venture a guess that it
will hand the extra heating needed. All you want to do is raise the
temperature of the space from say, 60 degrees to 68 or 70 degrees. That
does not take much energy. If you were adding a couple of new rooms,
requirements would be to raise the temperature from the outside air, say
zero on a very cold day to 68 or so.
More complex is how to determine the location and amount of heating surface
you need. Balance is important so you'd have the heat on all sections, not
all at one end of the living space.
If he has the headroom, this would be a perfect application for floor
based radiant heat.