I am not a surveyor. Cardiac Paul and some others are pros at this, but let me ansewr your questions here, yielding to any professional advice:
1) Always pull the boat on a survey. You need to sound the hull and see what you have under the waterline. I would venture to say, percentage wise, this is the MOST important piece of a survey.
2) Engine survey. Many people choose to do without a seperate engine survey. I think this is a mistake. A good boat surveyor is typically NOT a good engine surveyor. He may be able to tell you basics of whther your engine is headed for a rebuild... but not the depth a true mechanic engine surveyor would (and I cannot overstate this enough). On a boat that old, give about 12-15 k for a new block, I would get a sepearte engine survey unless the value of the boat did not warrant the 1000-2000 extra.
3) Offshore surveyor. You need to find a good surveyor that is very knowledgeable on boats. If they have done surveys of your specific make/model, that is even better. Really good surveyors stay booked solid and will tear a boat apart. That is what you want. You need someone that is VERY skilled on large sailboat surveys. As far as the offshore piece of it... nah, just follow the advice above. Most surveyors are not previous career circumnavigaors... they are just damned detail oriented and KNOW THEIR BOATS!!
4) Missing something important? Well, that possibility is always there, even with the best surveyor. Can you sue them? Maybe. Would you get anything? I doubt it. You are paying them for an opinion, not a omnisicent knowledge of your exact boat. That is why it is important to get a good surveyor... even if you have to wait and pay more.
5) We already discussed the engine aspect, which is seperate. A survey is a survey... and that is a FULL, in depth, survey from top to bottom. Do it, do it right, do it right the first time.