I have installed a 5K BTU window unit in my C27. It fits in a gasketed hole in a bulkhead between the cabin and the port lazarette. The lazz hatch remains open right now for cooling, but I plan to add an active air exchange system to allow the laz hatch to stay closed. I had concerns about condensation drip, and was ready to plumb a line to a shower sump that is just below it, but so far it is apparent that the condensation is being carried out as vapor on the hot side.
The 5K unit works pretty good for me so far, working inside the boat inside a non-insulated workshop. I think having more BTU will be better for me though.
The biggest restriction I have is height. That was what drove the purchase of the specific unit I used. After finding my 5K unit, ($99.00 at WalMart) I found a better unit from LG, specifically the LG LW8012ER. It is an 8K BTU window mount AC, so it would instal similar to what I have currently. All I would need to do would be to modify the dimensions of the hole in the bulkhead somewhat.
I believe based on my experience so far with the 5K unit that the 8K unit would chill down the whole boat quite nicely so I have gone ahead and ordered the 8K unit today. It is available a Walmart.com for $218.00