Since the return temperature is 140-150 I would not envision a problem with a boiler living 20 years. As far as the dew point goes and condensation in the boiler at such low temperatures in a non condensing boiler those worries are attributal to hot water boilers.
Steam boiler generally do not have much air mixed in the condensate return water.
The key for the lower temperature is the cast iron radiators taking all the heat out of the steam and causing the condensate to be at a lower temperature.
Some times getting to technical you create a problem that does not exist.
Make sure you use a good boiler water treatment for your boiler. Follow all the appropriate instructions for the chemical treatment installation. Some times to much chemical is installed and the boiler water will surge.
This problem is easily corrected by draining water out of the boiler and adding more make up water until the surging stops. The time to do this is before the heating season starts.