Mercury water pump replacement

[deepsushi]

In the process of rebuilding the water pump on my 4 cylinder 115 Mariner (Mercury). The motor did not want to pump water while running on the hose in the yard so I figured it was time for a rebuild.

I managed to drop the lower unit with only a minor amount of wrestling and swearing.
The impeller is not too bad -- but definitely due for a replacement. Mercury sells several different rebuild kits and I am not sure which kit to go for. Do I want to replace the whole thing (upper / lower) or just swap out the impeller / gaskets? There was a lot of corrosion around the lower pump body bolts and the face of the lower appears to have some corrosion on the surface.

Any experience / recommendations with the Mercury OEM vs the Sierra aftermarket kits? The sierras are about half the price of the Mercs.

Also, the water tube gasket which connects the pump to the water tube was shot -- are these typically included in the kits?