Propane Gas Boiler Replacement

I am seeking some assistance with replacing a propane gas boiler. The current boiler is a Hydro Therm HI-125B, 125K BTU, that was installed in 1988. It is for a home that is used minimally in the winter, during which time the thermostat is set to 50 degrees to prevent pipes from freezing. I have received five estimates from three different contractors to replace it with either a similar boiler or a high-efficiency wall-hung boiler. One of the contractors raised questions/concerns about venting for both systems 1) for the high-efficiency the pipes would need to extend approximately 25 feet to the outside and there apparenlty isn't enough space in the ceiling to get the required angle, and 2) the current boiler vents through the chimney, but the vent hole would need to be raised to accommodate a more modern boiler which is much taller (the current boiler is 24" high and the bottom of the vent hole is 43"). The two other contractors didn't make mention of either of these issues and are strongly suggesting the wall-hung option. In follow-up conversations, one contractor indicated the wall-hung boiler could be vented through the chimney. I have no idea how to determine what the best option is. Any guidance would be appreciated. Thank you.