The PSE web site seems a bit glitchy today. The rebate application didn't want to load on my browser.
They have some heaters that I sell that get the $50 rebate, those are the ones I'm mainly selling for gas.
Which link do you have the the tax credit models. Energy Star Tax Credits
If the tank is eleven years old, it's hard to say about the condition of the tank behind the outer painted steel liner. There could be moisture that is dripping onto the pan, or you could have a tank leak on the inside. Most of the time, if someone is in doubt, you are better off being safe and replacing after 10 years.
Many condo associations will require replacement every 10 years to prevent damage to the other units.
There are several types of gas, oil, and propane water heaters:
1) Storage tank. There are NO residential storage tank water heaters that can qualify for the tax credit because none can meet the Energy Factor requirement of .82. However, there are some commercial storage tank water heaters that can qualify for the tax credit because they have a thermal efficiency of greater than 90%. These models are larger than what is typically considered a residential unit and may not have the standard safety features of residential models.
2) Tankless. For a partial list of tankless water heaters that qualify for the tax credit:
Converting to tankless runs about $3,000 or more depending on model.
Most gas storage tanks that don't meet the federal tax credit can be installed in the $1000 range.