My .03 (inflation:})
Being of the type that believes in making anything I can rather than buying, you can guess my answer.
BUT there is another side to it also. As your experience has taught you, building means that when you have a problem, you will know HOW the machine works and is made. This allows you to make the repairs quicker. Think of it this way, if you can involve your dad in the building, he will have a learning experience and this may pay off in the future when things need fixing! Also, think of it as a "giving back" as I am sure your dad like mine tried to teach me many things (wish I had listened better)
But good luck however you decide.