A seakindly design, intelligent engineering & solid construction. A hull that doesn't pound, & takes a punch. A soft ride reduces fatigue.
A hull shape that can track upwind. There is plenty of windward sailing offshore. Close windedness is less important than the ability to track well while making little leeway.
A well-balanced hull & rig that respond well to self steering, wind vane or auto pilot.
A keel that can survive a grounding and a skeg to protect the rudder. Sooner or later you will hit or snag something.
A sailplan that is easy to handle yet can be tweeked for better performance. The 2 biggest secrets of cruising is there is more light air than heavy, and cruisers motor more than they should. A light boat is not the answer a generous rig is.
A deck layout that keeps you secure when going forward in a blow. no matter how many lines are led aft, you will have to go forward. Bulwarks, tall stiff stanchions & lifelines, & well placed handrails.
A comfortable and seaworthy cockpit. This is where you spend most of your time @ sea & port. Good visibility from the helm, easy access to sailing controls, benches long enough to stretch out on, well-angled seat backs, and bridgedeck.
An interior arrangement that works for you. Privacy is important but ventilation, light sea berths, and functional galley are much more so.
As for performance & stability ratios there is no doubt that sail area-displacement, displacement-length, ballast-displacement, are useful when determining an offshore boat. However these ratios are so distorted on an older used boat that they are nearly useless.
CONSTRUCTION & ENGINEERING
Smart engineering and solid construction often (but not always) go hand in hand. a boat with massive bronze deck fittings, a husky teak boom crotch, & solid bulkheads may be well built but poorly engineered, who needs all that weight. Heavier does not mean safer or better.
PREFERENCES Fiberglass is a preference over steel, or aluminum, (less maintenance) Composites are also very good, not yet as common as glass.
Solid uncored hulls, below the water line. Above the waterline coring is good, huge weight savings.
A hull to deck joint that doesn't leak.
A laminated hull to deck joint over a mechanically fastened one, they are harder to find. Many production boats have joints that are through-bolted as well as chemically bonded.
A hull with transverse floors & longitudinal stringers instead of liners & molded pieces.
A boat with internal lead ballast.
A deck-cored with water-resistant material & solid laminate in load bearing areas. Most boats are balsa cored, this may or may not be an issue in an older boat. Some boats have plywood cores & these are almost always a problem.
Great men always have too much sail up. - Christopher Buckley
Vaya con Dios