I have an Ultralight 403 not an eVolve, but I can give you some information you want...
Drag: None when you remove the eVolve from the drive well (you cannot mount the eVolve on the rudder of a TI). None with an Ultralight (see pic below) as it raises out of water, takes no storage space when not in use and you can peddle with it in use if you want.
Corrosion: All plastic and stainless - Out of water when not in use.
Range: ULRALIGHT RANGE INFO
This should be about the same for an eVolve. You can also purchase a 520 battery made for the Torqeedo Travel 1003 and greatly extend the range for either the Ultralight or eVolve. I only used mine once, but am pretty sure I saw the range reading 20 miles + at something less than full throttle. It would run at around 4 mph if I held my hand in front of the shaft acting as a foil. It may do better in smoother water.
Maintenance: Other than the rinse you mention, dielectric grease for the electrical connections occasionally.
Here is a pic of the Ultralight mount I fabricated for my TI. I replaced the UL with a Travel 1003 so this setup will be for sale when I get around to posting it.