The gaskets (if there are any) will be inside - I dnlt know this engine, but as a general rule there are four things pass through the leg - drive shaft, cooling water supply pipe, exhaust and gear shift linkage.
Needless to say the prop shaft has an oil seal on top of the gearbox, and will spline into the bottom of the engine. THere are variuls ways of sealing the water & exhaust pipes, but you should get all the bits ytou need for the water pipe in the impeller kit.
Some mercs used a rotating gearshift, which just splined together, others you have to disconnect the shift shaft at one end. Youre manual will explain in detail.
The one screw that usually catches new people who spend hours trying to free the gearbox from the leg is the one hidden under the trim tab (behind the prop).