For a machinist it is really fun to build these bikes. The possibilites are endless. I'll stand at the mill ,running production so I can stay alive, and look across the shop at a project bike...wishing I could mess with it instead of doing what I'm doing.
I think you'll be happy with the I-motion 3-speed. From the information I have gathered; the number one reason for failure, (in any of the multi-speed hubs), is a mis-adjustment of the shifter. The little wire rope, when pulled, moves a slider that engages the planetary set. If the cable travel isn't precise there will only be partial engagement.
Once you are able to start riding check the travel periodically; in-case the main cable stretches.
I'm sure you'll have no problems at all...it's just the nature of things...machinists tend to have a knack for making things right, and keeping them right...