I know a lot has been said already regarding the 18-105, but I wanted to say a little more. Someone in your situation, having the 55-300, may at first think to choose the 18-55 over the 18-105 with the reasoning that having a range out to 105 is too much overlap with the 55-300. You can nudge this discussion either way. I have both, and without a doubt, I would say don't worry about the overlap and go with the 105. The 18-105 is a great range for a LOT of day to day photography. You may not want to carry that 55-300 everywhere or you may not want stop and change lenses just to get a bit more reach over the 55. When the dust settles, you will be mainly using the 55-300 when chasing birds or other wildlife or sporting events, etc. The 18-105 will give you a more handy range over the 18-55 for day to day pictures. Nothing wrong with the 18-55, just added convenience for always having a little more zoom with you if you choose the 105. If you really don't think you need the added range of the 105, then consider a faster lens like the 17-50 mentioned above.
As for macro lenses, they are certainly fun, but you might not need one for flowers. You can get a good flower shot with the 18-55, 18-105, or even the 55-300. Unless you are looking to get details of the bugs inside the flower, hold off on a macro until you know.