You'll probably get a ton of answers to this one, because I think it's in the range where all the varying levels of paranoia will work. The fact that they kept turtles isn't a big deal...other than that the tank probably had much higher nitrogen levels. In fact, that may work to your advantage as diseases probably aren't nearly as communicable from the turtles to your fish.
One big question is, are the plants live? If not you can treat them just like the gravel, if they are live you're going to have to be much more careful.
A bleach soak would be fine, and would probably be the highest end of response. Just be sure to soak/rinse everything multiple times afterwards with first regular water and then water with dechlor (run it at a higher than normal concentration and it should get rid of the bleach). The filter itself and the gravel would be fine after this treatment...the media itself might start to disintegrate, and will be much harder to get the bleach out of. I would just buy new media, rather than trying to clean it with bleach.
Personally I think I would probably let the tank dry for a week in the sun, then clean it with vinegar and water. If it's been dry for a long time I might skip straight to the cleaning. You could use a little soap, but you'll have to rinse it incredibly thoroughly, then rinse it even more to be sure it's safe for fish. Most people will probably say to just avoid the soap entirely, for good reason.
