Correct. The heater may be okay but you have a leaking pipe. Improbable, but possible.
Seven years can be about right for an economy model in harsh water conditions. If the capacity has been adequate, replace it with the same. A larger capacity model does not last longer and may introduce connection issues (the pipes are in the wrong place).
For a traditional male with the right tools, it's a DIY. For wimps, metros, the aged, the infirm, those too busy, or for those who know the names of more than about six colors, it's a pro job.
Water heaters are commodity items. The same model is the same model. Installation is non-tricky - it's hard to believe even the fools selected by HD could screw it up. After removing the old heater, you set the new one in place and connect three pipes. You're done.
Too late, now. How often to drain (if ever) depends on the quality of your water. Study on this later.
Several. You'll need at least three (kitchen, bath, and laundry). This usually means running new electrical service to hard-to-reach places.
If you can live without hot water until your husband returns:
Drain the hot water heater:
- Turn off the water supply valve to the water heater. There are two pipes attached to the top, close the valve that is found on one of the pipes.
- Turn the heater's gas valve to OFF
- Hook up a water hose to the outlet found near the bottom of the water heater, put the other end of the garden hose outside.
- Turn the valve that's built into to the place on the water heater where you hooked up the hose.
- Open any hot-water faucet (this allows air to enter the tank).