The Defever 49 has a lot more room to live aboard and typically has twin engines. The KK 42 has less room, and a much smaller fybridge, and one engine, so it is more economical to run. The Nordhavn 46 was designed as an offshore boat and was built too much more stringent specifications than the Defever, thus they are usually more expensive.
I think you are not sure of the word refit, each year the boat will need maintenance, zincs, varnish, engine work, waxing, bottom paint every other year or more.
When you first purchase the boat, many things should be done, usually oil and filters, clean fuel tanks, new hoses and belts, batteries, maybe canvas and whatever is found on the survey.
We think of refits as rebuilding engines, new paint, fresh interior, replace teak decks, major stuff like that.