A lot of this depends on what you're doing. I was living on a 1970's fiberglass sailboat in saltwater. Upkeep is pretty low from a living aboard perspective. You need to have the boat hauled out and have the bottom painted about every 2 years + some other small things relating to the motor.

But generally fiberglass boats are pretty bulletproof.

Wood needs more care and I wouldn't want a steel baot for a number of reasons(maybe aluminum).

Are you interested in a sailboat or just a liveaboard boat?