I am coming over from the pleasure/cruisers boating side where I had 700+ hours on my engines without rebuilding them.

For a speedboat, how many hours before a rebuild is to happen? I am in the market for a used speedboat, but I don't want to buy something that will be a project (I have minimal mechanical ability). Should I look for something that has 200-300 hours tops? Is 400 hours going to be problems?

That's going to depend on how the engine is used and it's horsepower. For me anywhere from 60 to 300 hrs. You might be able to do top end only if you don't wait too long. Leak down test before buying. Should be a part of the marine survey.

What kind of boat and power?? 454 / 502 mags have gone 1000 hours on many occasions. 1200HP blower motors, not so much

I'm looking at a 496 HO mag right now. Is 400 hours nothing for one of these engines?

Yes.

That wouldn't scare me a bit if it's stock and well maintained. Should have plenty of service left in it.

They should last at least twice that many hours as long as they are maintained properly and not abused.
The PCM's can be scanned to see how hard they were driven.

I just sold 27 Fountain with 800 on 496ho ran as good as new. Don't be scared.

My recently bought VR had 378 hrs scanned on the stock 496 HO. 162 hrs at idle to 1499, 73 hrs 1499-2499, can't remember the rest but only .8 hrs at anything over 4500rpms. 155psi cold compression on all cylinders and 10-12% leak down cold. I would not be afraid, but DO perform the above checks before buying.

Just shy of 400 hours on my 496. Runs like new. Has never touched a drop of oil.