You sent me a more detailed email about your goals for the boat which include an offshore passage from Maine to Florida. Here is my response. There were two companies that built boats called Mariners and both companies built 28 footers. There was an Taiwanese company and a New England company. The Taiwanese boats were poorly constructed and used low density ballast. They would not be suitable for offshore work and frankly they are not very good coastal cruisers.
The New England built Mariner 28 is a nice little coastal cruiser. They sail reasonably well and have moderately good speed for their era. They were better than average in terms of build quality. They have a simple but workable interior layout. That is the good news. The bad news is that these never were intended to be offshore boats and they are now something like 15 to 25 years old. I would say that it is like this. If you went through and prepped the boat for heavier conditions, tying down locker lids, replacing standing rigging, making sure that you had good sails and running rigging, and an efficient way to reduce sail (roller furling does not count) and so on, you should be able to hop your way down the coast if you pay close attention to your weather windows. There are several long passages which require a fairly long exposure (the New Jersey coast comes to mind) that will require that the boat be prepped for several days at sea in harsh conditions. Obviously, this trip could be a cake walk, but if its not you need to be prepared.