The tier one guys will do a good job of preventive maintence on the boats. It is in their best interest to keep them on the water generating revenue . Because they are on the water all the time. Thats the rub, the boats have close to 100% duty cycle and things DO wear. Diesel, winches, pump motors, blocks, lines, will wear.
In a lesser way it's like buying a well maintained rental car.
An the point about charter setup is valid, it going to have MAX staterooms and minimal gear. The gear part can be addressed, the stateroom part is something you have to factor in. Probably the world most popular charter boat is the Moorings 50, which is based on the Oceanis 50. That thing has 4 staterooms and FOUR heads. Who needs that in a private boat? The current trend is to maximize public space and cut down on the private sleeping and wash areas.