I am assuming that you are not an investor bu actually wants to live in this house. As eluded to by my esteemed colleagues, basically you have to be prepared to be a landlord for at least 1 year.
Before finalizing the deal, make sure you do your Due Diligence on the lease agreement, and the tenant. Review the tenant's payment history, his income, and whatever information the current owner has on them. Like Bruce said, ensure to get estoppel certificates.
If the tenant is NOT a good tenant, you may end up with a possible eviction and all the expenses associated with it. If the tenant is section 8, you might want to check with SHRA on how to terminate the lease, after it expires, etc. If the tenant is a good tenant, just make sure there is a common understanding that you will not be extending the lease once it expires. Alternatively, you may want to incentivize the tenant to break the lease. Who knows, perhaps for 1 month free rent, they may want to leave earlier.