I am in the market for a good used self-propelled gas mower but would really like it to have electric start. Is it possible to add this feature to a gas mower? How complicated would it be?

Hi,
It is possible. Not going to be cost effective, however. Depends on the make and model engine as to cost. You could figure a kit would be $150-180 as a general rule. Might be found discounted. Batteries average about $60.
Could possibly be found on a junker, blown engine, bent crank, etc.
Peace,
Clarke

Although there is another "electric start" option usually reserved for snowblowers, where you take a 120 V extension cord to start the engine.

Crigby is on the money for kits.