Engines:
I don't know enough about the engine brands to make a worthwhile comment on Merc vs Crusader. My general understanding, however, is that the level of preventative maintenance that any given motor has received over the years is critical in determining its useful life and likelihood to break unexpectedly. Look for documentation to prove that regular maint. was done. Don't take someone's word for it.
A/C:
When the A/C died in my Trojan I considered all options, as internally installed units are EXPENSIVE. What I found is that no portable option worked for me. They were either too bulky, mounted in a way that wouldn't work for me, didn't produce enough cooling, or all of the above.
Why would you want a portable unit? I was looking based only on price. Do you have any other reason why a portable unit would be better for your situation?
Survey:
I purchased my first boat (36 ft. Trojan) without a survey and DEEPLY regret that decision, as the list of problems that I overlooked is lengthy.
My current boat had a full survey and sea trial from a reputable surveyor that I chose (definitely NOT the one that the broker recommended) prior to purchase. I've still had to fix some things, but at least I had a better feel for the scope of repairs that would be needed.