About the speedometer. You'll likely want a sonar. I did, so I got
myself a fishfinder ... on sale of course. Combines sonar, speedometer
and water temp all in one (discontinued 300TX model). Sure, GPS is more
accurate as a speedometer ... but doesn't tell you the water depth :-)
The speedometer is useful in that when combined with the tach, helps
you optimize the trim setting. And the "out" switch will work just fine
for that ... used mine a few seasons before fixing the trim switch.