You could use Google Earth the way you described, and the maps would be great overlays if you wanted to put them back in Google Earth as KML/KMZ files, but as you said it would be at best +/- 3 meter accuracy at best. I would suggest using the website below and either get a true elevation benchmark form an Engineer and Municipality or use the elevation from the site and run an auto-level. You can get elevations for all of your points from the site, but I haven't tried that so I don't know how accurate it would be and how it compares to Google Earth. Makes sure you set it up to give you the decimal DDMMSS so you don't have to do any conversions besides the ft to m elevations.
Things like manholes, inlets, corners of concrete structures and tract property fence posts (the bigger ones) are all great markers that should stand the test of time. Just make sure you pick the bottom of the post.