I am not a mechanic, so treat it lightly:
1. Front seal - can be replaced without moving the engine out; i would, however, take a good look if it is really this place as there are quite a few spots in the front that can leak
2. Head gasket - I would, again, double check if it is really this part and not something else; if it is, I would try to re-torque, as suggested; head gasket replacement can also be done without moving the engine
3. The rear seal - this is the tough one. i would make sure it is engine, not gearbox leaking; if it is the engine and the leak is big enough that you cannot live with it or hope it will stop leaking, the repair is major if I recall correctly and may require removing the engine from the car.