Electric start next best thing to sliced bread! I use one group 27 battery and a 55# TM can fish all day and it will still start the motor. Even if you did run the battery dead you can always pull start it but never have in the last 6 years.
Remember to include the price of a flywheel in the cost, if you need one. I have converted several small motors to electric start and I think almost all had to have the flywheel changed. Another problem I encountered is model years and starter gears can be different. The gear in the starter must have the right amount/spacing of the gear teeth. I.E. an 88 johnson has an 11 tooth starter gear where a 87 or earlier has only 9 teeth. This is an example and fictitious as I can't remember when and where the change took place and Merc may not have an issue.