Hi guys ... im buying a 2000 Volvo in a week. I have the option of picking one with a Cummins Engine or one with a Volvo Engine. Both Engines have about 525,000 miles (910,000 Kms) on them. Which one should I go for. How long do you think they will last?

There is no set answer here. It depends on how the motors were maintained before you decided to buy. Either way, prepare yourself for an overhaul and you may be pleasantly surprised.
Even if a motor had a great maintenance record, you never know. 1/2 a million miles I would ask for all the maintenance reports and any oil analysis reports that were available. I don't mean to sound negative, but these motors have just enough miles to give you headaches, or they may go another 2 or 3 hundred thousand miles.
Good luck to you!