No you can make them oil/water tight however it can be a labour of love. Common leaks are gearbox/transfer box so the cost may not be high for a couple of seals buy the work is considerable and without guarantee.
Steering boxes can try the patience of a saint but it can be done, next up rocker cover gasket, or sump, and sometimes the oil cooler pipes can be a bit damp, the lift pump gasket (and even the pump) can leave a drip here and there. Even the swivel oil can be replaced with 'one shot' grease helping out the tired swivel seals.
Coolant leaks are the same as any other car but the 300tdi has a second gasket behind the water pump (P gasket) and can be a pain.
Work out the area where the leaks are coming from and try to deal with them in an order that suits your finances at the time, it can be done but you must persevere.
regards
Dave