Amateur responses, please feel free to double check...
The advantage of traditional external hinges over a piano-style hinge is that the door can be removed more easily, and their bearings can be repacked in case the door ever sags. I don't know if repairing a sagging piano-style hinge is practical. This is not the sort of repair you should need to do every year or even every decade, but it does happen eventually.
Bolting to the floor is important for security, period. If you're bolting into concrete, then there's almost no loss of fire protection at all. In general I wouldn't worry about it unless it was an upper floor installation, i.e., there's a good chance of fire below your safe. Otherwise hot gases are unlikely to flow along the floor.
Regarding moisture protection, few safes are watertight until the fire seals heat up and expand. Adding another hole shouldn't make much difference. Address this with a dehumidifier and/or heating rod, and you should be fine.