We went with built in, placing a 10K webasto under the V-berth and a 6K in the aft berth, as our 1989 didn't have anyplace to mount the unit midships, and no real way to efficiently run ductwork forward to aft.  It works great for us and a 2000 generator can run either unit (but not both).  While I was doing the installation, I placed a window unit in the companionway with a plywood hatchboard to make the cabin liveable at the dock (and to be able to work at a comfortable temperature during the installation).

We gave up some hanging locker space to fit the ductwork, but it was well worth it, as the reverse cycle system gives us year-round comfort.  I've been (slowly) working on a writeup of how I did our installation, hopefully I'll have it finished soon to share with the association for any other MK 1 owners looking to do a similar upgrade.

Hi James,
I installed a Webasto 16000 BTU unit in our V-berth last year. We have one vent register in the v-berth starboard locker and the other register goes into the main cabin via the same locker but the opposite side. I glassed in a shelf for the unit to set on. We purchased the complete kit that had everything needed to install it. It has worked great in the hottest days and the coldest nights. In North Carolina we get a bit of both.
 Tim