Since this is your first purchase it would be advisable to read the Pennsylvania Landlord and Tenant Act of 1951 Link
To expand what @Thomas Page said about waiting for the lease to expire here is the text from the Act that I believe is relevant.
With that being said I would request the lease from the current landlord before purchasing. If there is no lease then the tenants are on an assumed month to month lease and would require a 60 day notice before you can raise the rent above 4%.
Also if you are planning on raising the rent over $500 don't expect those tenants to stay around very long. If they are elderly they are most likely on a fixed income and won't have the means to pay your new rent and subsequent late fee's etc.
Your best case scenario would be the tenants are on a month to month lease, you notify them of the upcoming rent increase and they leave on their own in 60 days. Otherwise if there is a lease in place you will not be able to realize your "best case scenario" until that lease term is up so plan accordingly.