1. The reason why you get the Mobility Boots and Ghost is to make a faster "fling" gank, which means that u throw over the enemy. It can be done to realy fast ganks at early level and easily gain yourself Firstblood.

2. When should i gank? Well, actually hit lvl 5 is the best level to gank atleast by my opnion. Than you're W will make alot of damage to the enemy and if you have a good dps like Graves or Lee Sin at your back, just fling and sitback while you assisted your team to the Firstblood.