...All you are going to do is sort out which boats aren't worth looking at. After that, it is up to you and your surveyor. If they won't let you nose around pretty good before the survey, walk on.
Exactly. How else do you weed out the potential from the junk? There are so many boats out there, and unless you’re budget is unlimited, you have to limit your search in some way. So of course ask lots of reasonable questions, especially if the ad is lacking (which is sometimes enough of a sign to discard the boat).
And unless you’re rich, surveying every interesting boat is simply not on. Heck, even the initial visit and inspection will cost time and money. Again, unless money is no issue, most of us can’t afford to waste it. Of course sellers will obfuscate, but few will outright lie about very specific questions. Usually (not always), how people answer questions tells you more than the information they provide.
Of course, not all sellers will know everything about their boats. And unfortunately most brokers will often have no idea. But in the former case an honest “I don’t know” is still a useful answer. When a broker didn't get answers to my reasonable questions, then this boat quickly got removed from my 'possible' list.
I don’t think I’d be asking about oil change or impeller schedules, although I suppose a clear answer on that gives you a good idea of maintenance quality. Bottom paint, engine hoses, heat exchanger … all part of a more detailed inspection and possible survey. I asked about state of the boat, and its history. How old is the standing rigging? How has the boat been used (raced? Circumnavigation?) Have there been any hard groundings, flooding/sinking? Are there any deck or hull leaks? How many hours on the engine?