Our Time, as well as many others, had serious problems with the XDP drives. VP should have replaced every one of them at no charge to the customer but they elected to offer replacements at a reduced cost and still charge the customer normal labor rates to make the change. Not a very popular solution to a problem that they are responsible for.
This will probably stir up a hornets nest but I'm not going to tell you not to buy it. But I will tell you that if you do just remember not to turn hard left or right or run it with the leg raised more than 15-20 degrees. The U-joint on the drive shaft will rub on the bellows, when put in any stress, and cause damage to it. The boat that your looking at probably has the 4.3L engines, I think the bulk of the problems with the XDP's were those connected to the higher torque 8.1L engines. You should be able to buy the boat about $30K under market price (that's about what it would take to replace the drives with the latest and greatest). Have them pulled and checked for water intrusion, U-joint damage, etc. Have the latest version of bellows installed and who knows, you may be fine. Be prepared to take a beating when you sell though.
Boating should be a kick-back "PleasureTime"
'06 Regal 3360 5.7OSXi XDP