For such small fans, ID/FD cost wise difference is marginal compared to boiler cost. For external fired and small boilers, you can't make air tight furnace and if it is nor air tight all hot gases will leak into boiler room. This itself is enough to prevent FD use. ID draw backs are not so great...
You can keep about 500,000 BTU/ht/Ft2 heat release rate on Grate and try to maintain about 2.5sec residence time. Vol release rates could be less than 18000 Btu/hr/ft3.
What's the Boiler capacity? For small boilers you don't need all that. Generally Chimneys are designed(Thermal) to create small negative draft at the inlet unless it is meant for creating some draft for boiler. Minimum Temp should be to take care of above and any acid condensation.
