I have an Webasto 3900, it draws it's fuel from the diesel the engine runs on downstream of the 1 micron fuel filter and has a shut off valve to isolate it just in case I need to. The max fuel consumption for my Yanmar is 1 gph so it's not likely to create enough suction to create problems for the heater.
If you draw your heater air from outside the cabin, as it heats it will absorb more of the moisture from inside the cabin. If you draw your heater air from inside the cabin, you get warmed air, with little additional moisture absorption capability. The heaters that run a stack pipe out the roof make it warm, but don't do much to dry the air inside, just warm it.
The greater the temperature differential between outside air and cabin temperature the more moisture it can absorb.
If you need to dry clothing and the interior of your boat, forced air is superior to hydronic and heaters that just heat the air inside the cabin. Firefighters and pilots have been trained to recognize the effects of dew point and how temperature increases change the ability of air to absorb moisture.
Just a point to consider... My Webasto is mounted inside the cabin under the sink, and while I can hear it run it isn't in any way annoying. I often leave it on all night, the clicking of the fuel pump is it's most significant noise, and it's not much.