cant help you about the benny, but being in a similar situation with a Moody, I reckon the best way forward is the hot water circulation rather than blown warm air. much smaller pipes therefore easier to run, and generates hot water in port or when the engine is not in use. cost doesnt seem much different.
I have a webasto, very recently installed on a Jeanneau. You might tract the manifacturers route by looking for cut outs in the boat woodwork. I did and didn't like the route. You can get the pipes into some amazingly small spaces. I would Phone Keto marine and get one of their guys to have a look fiorst. They charge so little to fit it really is not worth the hasle doing it yourself.
Thanks for the various suggestions. Ozzyb had got me a bit confused. We are buying a 2000 boat with twin aft cabins and I am sure the galley/chart table is to port and heads stbd.
If you go under the cooker the next stop is the fridge which seems to have very little space undernaeth. Thus I wondered if to stbd behind the heads is a possibility but the plumbing seems to take up most of the space.
I've just had a 3500 installed on my 361. Heater is on stbd aft locker bulkhead. Hot air towards topside at the quarter. Ducting, high quality 75 mm, makes a very tight 180, turns back on itself and through port aft cabin. There's plenty of room on this side. Take a look under the woodwork of the stb cabin and you'll see that there is very little room. No point using small diameter ducting. At the moment I just have one outlet from the aft cabin. The route forward from the port side cabin to the saloon looks torturous and I don't want to let just anyone with an electric saw lose on my pride and joy. Don't have the courage to take it further just yet! Leave the doors open and it soon warms the boat!
It's not a big deal to run the fuel line from the tank to the stbd aft locker. Ditto the power harness. My temp controller is near the 12v switch panel. The temp probe is stbd side saloon, wire runs through the heads. No problem with room for a single wire.
I'd be interested in anyone knows an easy route from port aft cabin to the saloon?