Well, I believe Garp took him to Dadan because Shanks "inspired" Luffy to become the King of Pirates. Otherwise, Garps would probably have felt Luffy was been nicely taken care of by Makino and the mayor. Now, would that turn Luffy into a Marine? My guess is not. Shanks was a great inspiration and role model for Luffy, in the sense that made him less of a prideful with stuff that doesn't matter (like being thrown drink), and, most important to value friendship above all else (which then prompted him to force himself into be friends with Ace, and, later in life, with all the Straw Hats). But I truly believe the will to become the Pirate King was not inspired by Shanks, it was just a coincidence that he was also a Pirate. Luffy wouldn't have reminded Shanks of himself if he had simply copied Shank's dream, or if he had spouted something that he heard somewhere else. It was the sudden and genuine declaration of his already formed dream that moved Shanks into making the promise (This is also inspiration for Luffy's promise to Laboon, btw).
Luffy, in my opinion, would still be quite care free and very genuine, but maybe a bit more prideful. He would value the niceness of people, but maybe not to the extent that we know today. He probably would be a very different kind of strong (specially since he wouldn't have eaten the Gomu Gomu fruit). He would be a very powerful and amiable Luffy, but he wouldn't be the Luffy we know, and he wouldn't be Luffy at his maximum potential. IMO
