[jrref]

It's not common for the pressure release valve to be leaking unless there is excessive pressure in the tank. It's a very common mistake to replace the pressure release valve without checking the whole system first. Also while your hot water heater can possibly last 15 - 20 years, after the warrenty period you are gambling. It could fail at any time in many ways. In this case the owner could have changed the tank at 6-10 years for example and avoided this platform repair job. In some cases the leak can cause mold and that compounds the problem. So at the end of the day you are going to pay either way and you might as well change the heater at regular intervals and avoid a costly and involved repair or even worse a huge flood. As far as getting a new heater. I would recommend getting a quote from the local guys and Home Depot. Home Depot uses a very good plumbing contractor and the whole process is very simple.