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.
Good luck with it,
Frank_S