Quit to sail
A lot of this depends on how knowledgeable you are about boats, the sailing venue you plan to sail in and the nature of your general preferences and tastes. The traditional rule of thumbs that you used to see in print suggested that you need somewhere around 2 1/2 to 6 (long) tons of displacement per person to go distance voyaging. Today with more sophisticated deck gear this limit has been extended upward to maybe 7 or 7 1/2 or so (long) tons displacement per person but here is where preferences come in, because I personally would prefer to stay closer to the lower limit while many would prefer to stay closer to the upper limit.
Displacement is a much more useful yard stick for sizing a distance voyager than length because displacement will (by and large) give you a sense of how much volume and carrying capacity the boat will have. All other things being equal, once you have chosen a displacement range that appeals to you, then in a general sense a larger boat in that weight range will offer more seaworthiness, speed and comfort, albeit often at a higher price.
How much maintenance should be budgeted is like asking how long is a piece of string. Complex boats with lots of gear, and older boats in mediocre condition require the most maintenance. Simple, well understood and well maintained boats (as odd as this sounds) require the least maintenance. In a loose sense, most boats cost between 10% and 15% of thier purchase price per year to maintain but, of course, there is a wide variation from owner to owner and boat to boat.