Brands is great but if you're in Brooklyn go to Roy's. They know what they're doing too and you don't want to drive out to LI every time you need service. Local is better.
Again, this depends where you live.
As far as renting, the places that rent will cost 100-150 for the weekend for a $1500 or so bike. It won't be worth it unless you're rebting while traveling. Better off finding someone to lend you a bike for a few rides to try it out. It's also very important to be fit properly for your bike which you won't get on a rented bike (or a borrowed bike but at least that'll be free).
Keep in mind this isn't a cheap sport. Besides the bike theres the shoes, cleats, pedals, helmet, bike shorts, jersey (if you want to carry anything like a phone or food this is important), spare tubes, bike pump, saddle bag, GPS and mount, optional HRM, etc....
As far as a used bike, it's not a bad idea but you need to know your size going in. And not unlike buying a used car, you really don't know much about the bikes history. So unless you're buying from someone you trust (read: Not Craigslist) and even when you are, you're probably better off buying an entry level new bike at a shop that will size and fit you properly then provide free service on the bike for the first year (at least) for free.
