Re: looking for seal kit for lower unit??
Since its your first re-seal it will take you longer, but I don't see why it should take you more than a couple hours unless something is exceptionally stubborn. I'd set aside a morning/afternoon for it.
The impeller isn't hard to replace. Just set it in place on the driveshaft with the key in. Put the impeller housing on top and slowly turn the driveshaft or propeller clockwise as you push down on the housing. It should pop right in. You shouldn't have too much trouble lining up the water intake tube, but if you do you just won't get any water circulating. Then it will overheat and possibly destroy itself if you don't turn it off right away.
You will be greatly aided by a jig to hold the lower unit. Here are a couple photos of mine. It holds every lower unit I've worked on from 1949-1988 from 3-35hp; OMC and Mercury. Its made from a 24" piece of 2x6. The slot in the center is 2" wide and extends for 9" then tapers to meet at 11". I just have mine clamped with a large "C" clamp so I can remove it (not that I ever do), but if you put a lot of pressure on it you will need something more robust to hold it.