A lot depends on the strength of wind and size of chop generated, but provided that all of the bits are strong enough, the plan is sound. What you are planning sounds a bit like a slightly shortened Marblehead, but probably built a bit more solidly and therefore heavier. You are looking for pleasure over the years to come rather than racing and repairing. Its a good idea to combine the mast step and fin box into one strong unit - thats where most of the forces go. Thinking back, at one of the world championships held at Fleetwood back in the late '80's, there was a competitive boat called "Odi" with much the same construction as you have described.
Sealing the rudder linkage exit is easy - you can buy soft plastic bellows to do that exact job. Sealing the running rigging line exit is less easy because of the large travel needed coupled with the need for it to be free running so that it can pull out in light air. Perhaps a prop shaft going through the watertight bulkhead with a pulley on the end instead of a prop? Thats assuming a winch. A motor/gearbox and ESC would probably serve better than a big servo. Think cordless electric screwdriver.
Your servo choice would work, but is overkill. A standard servo will do the job.
Carbon rod blanks have been used successfully for masts, as has aluminium tube from the hardware shop and dowel from the same sort of place.
If the sails will never be stowed folded or rolled, mylar is great. Otherwise, cloth is the thing. Once rolled or folded, mylar remembers the unwanted shape for a long long time. You might want to consider a simplified, self tacking rig rather than a full on J class type rig. If you are doing your own designing and developing, large plastic bags opened out combined with Sellotape are good for experimenting cheaply to determine your favoured size and shape. A heavier boat, unless it has a much longer fin, will be a deal more tender than a modern racer, it will therefore need to run a smaller rig in any given conditions.
For standing rigging I like venetian blind cord, available in various weights and colours, running rigging I prefer salmon backing for its flexibility and very low surface friction. That, and I was given a reel of it many years ago.
Quite possibly you will find a shop that sells stainless steel shark trace. This makes excellent linkages and rigging hooks. Fishing tackle shops are also good for swivels.