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I towed with a 2004 XC70 D5 auto for 6 years without problems.
During that time I covered at least 20000 miles with either a 1500 Kg caravan or a one tonne trailer.
Don't worry about the auto box, providing the ATF is clean, it is perfectly suitable for towing. There is not any need for additional oil cooling or any other modifications, just tow and enjoy!
The only thing you need to do is check that the auto box is working correctly and that the ATF is clean before you buy it. If the car has done anything over 75 - 80,000 miles, it would be worth changing the ATF as this will help prevent problems developing.