There is no guarantee it will keep its PR. If it gets parked, the PR will go down faster. At the same you can fo from PR1 to PR3 in a mater of less than a month. Develop a good site that is bot friendly and you should not have a problem keeping or increasing the PR. Before buying, make sure it does not have a fake PR. At the top of namepros, click on Domain Services, and you will see a fake PR checker.