A quick tour of e-bay revealed several 12 V. power supplies for the Lenovo notebook: 0B47481 65W DC Car Slim Tip Travel Adapter is one.
I don't know what the required voltage and connector is for your specific laptop.
The low voltage cutout will not give you adequate protection. A 12 volt lead acid battery should not be discharged to less than 12.2 volts. (Sure an occasional deep discharge may not ruin the battery, but it will decrease its life and capacity). The Victron (cheaper are available from China) will monitor the voltage, state of charge, amps in and amps out. (There are more expensive options--or some cheaper options than the Victron.
The specs on the 2004 75 HP Honda give an amperage output of 16 watts--the amount used by the motor vs @ left for charging will vary with speed and conditions. But if you are pulling 5 amps for the laptop, it will not leave a lot for the battery charging--Also I am not sure about the charging profile on the 75--related to engine RPM--often one has to hit higher RPM to get the full output.
Not sure how you are going to measure the current output, but it can vary a lot, depending on SOC of the battery, temperature, RPM of engine etc. Also most Digital volt meters have a fused limit of 10 amps. There are other meters, either with a shunt (such as Victron or others) or a clamp on meter which will measure up to 400 amps (or more).
Good on the planning. To me plotting the course is the least of the homework. We often divert from the plan, as Colby found out with his travels this last month.
Great on a summer of fun on the C Dory--Jonathon Arthur made his first great loop navigating only with an iPad. His 22 was so minimal that looking at it, you thought that most likely it was on a day trip.
Here is a link to Pat and Patty Anderson's great loop blog. Toward the end he makes comments about the navigation.