I think the biggest impediment to your plan will be the physical characteristics of the ramps you intend to use. Most of the ramps I have seen simply will not support launching a boat that big: I understand you intend to play the trailer down the ramp with cables (and avoid the truck in the water scenario), but the ramp would need to be very long indeed in order to get the boat in water deep enough to float. I would think you would wind up with the trailer's wheels in the mud. I know you want to avoid using a crane, but that may be your only way to ensure launchability. And you might need a crane anyway to step the mast. I supposed you could rig up an A-frame device, but that seems to me to be just as big a pain as hiring a crane.
As for the difference in set up between a 27 and a 30: I think the 30 is worth the extra money. Its a great boat and really has significantly more room than the 27. I just don't see the 30 as "trailerable".