Re: 2000 115 Johnson SEI Gearcase Assy
If $150 includes them attaching the upper part of the shaft, then it's a great deal. All you have left is screwing in your old shift shaft.
If I understand correctly, in order to connect the upper to the lower, you still have to take out the lower portion because you can't get the pin in it. This requires removing the pinion nut/gear and pulling the shaft out. Is this correct? If so, you'd be better off using your existing shaft. You have to take apart the old one to get the shaft out, and take apart the SEI unit to attach the upper portion.
Ignore what I said above until someone verifies it. I would just call SEI and ask them what it will take and if they will install the shaft for you.
Good luck...I took a chance on a used lower and so far so good. But you can't beat SEI's warranty and price.