The main thing you need to look for is that the Garmin has maps for the entire North American continent, and the Puerto Rico, Bahamas, US Virgin Islands, etc.
You'll also want to make sure that it has voice recognition, Junction View, and for '13-'14 radios, 3D landmarks.
I've been using a 3590 for a couple of years for my maps, and I know the 3490 works well too. I've heard the newer 3495/3595 model work as well.
I'm not sure how well openly sharing maps would go, because the maps have data identifying which Garmin device downloaded them. It wouldn't seem hard for Garmin to blacklist a particular unit and cancel the lifetime update subscription for it.
If the maps were shared within a small, controlled group of people it would probably not be much of an issue, but I don't think openly sharing them with the public would be a good idea.
You can pick up a refurbished unit for around $100, which doesn't seem like too bad of a deal. The only feature that you will lose by updating the maps from a nüvi is being able to search for POIs by phone number - but I'm not sure how useful that was to begin with.
You can also get maps from the underground gps forums, but those maps are usually edited in some way. That isn't a problem with a nüvi, but our cars are very picky that the maps are perfect, and it's a 50/50 chance that they won't load.