The cheapo Chinese blown air heaters seem to work well. I know of several in lived on boats, used as backup heating. They seem reliable. What kills them is a power failure while hot, when they don't get a chance to go through their programmed cool down cycle. That seems to knacker the fan motor, or bearings. They are so cheap, a replacement is easier than a repair. These were 5kW units into a similar size cabin to yours and did not recirculate cabin air. Noise wise, you knew they were running, but didn't seem obtrusive to me. Since you already have a diesel supply, wiring, ducting and possibly a suitable exhaust, then fitting one should be relatively simple. Note that the supplied exhaust is not suitable and will fail the boat safety scheme. The supplied dosing pump must be used to ensure the diesel feed rate is correct. Lots of posts on the forum on fitting these heaters. There are a few gotchas, so ask, if you decide to go this way.
