Salt vs. fresh water really comes down to age and maintenance for most items, but not all. Salt water is certainly more corrosive than fresh. A friend took his aluminum boat from fresh water to salt water a couple years ago. The biggest thing he found was it is now impossible to maintain the hull paint the way he was accustomed. The salt water wreaks havoc on painted aluminum. We get spoiled sailing in fresh water. When my father took his fresh water boat to the Caribbean he realized having a raw water cooled engine was not a good idea and had to replace the engine after a couple of years. Standing rigging can last many decades in fresh water, likely 3 to 4 times longer than in salt.
My boat spent about 10 of its 40 years in salt water and there are a few things that show from it such as the painted mast. If a boat has been well maintained then salt water shouldn't stop you from considering it but if it has been left to languish then walk away. That can be hard to determine from a distance but you could ask for detailed pictures of the rigging, plumbing, thru-hull fittings, electrical panel, batteries and engine before making the trip to see the boat. These are some of the places corrosion shows up.