A diesel furnace is truly great addition to any boat. I live in the Vancouver, BC, area and I've had a furnace on each of my four previous Gradys going back to a '92 Explorer. On walk-around models they hang perfectly from the under side of the walk-around right by the skipper's right shin. Vent it into the cabin and it naturally defrosts the windshield with the hatch open. I have a 305 Express now, and it's furnace is mounted in the accessories area behind the port side wall of the lower berth, with its thermostat easily reachable (in the dark of night) from the forward bunk.
I have stuck with Webasto all the way having heard consistently better reports about them and never having had any trouble other than reluctance to start when the ship's batteries are low! I suggest figuring out what size/model you need then go up one size. In my 305 I have a Webasto, Airtop Evo 55. I think I could melt a small iceberg next to me if necessary!
Beware of the Planar brand - made in Russia, less expensive, but apparently poor quality, and no parts these days!
FH