One more point on the use tax:

If you have to pay it anyway, why would you want to put it off until your
plane was complete, when it will be taxed based on it's value at that point?
If paying up is inevitable, you're certainly better off paying when you buy
the kit.

Also, I can confirm from my own experience that property tax isn't due on
the plane until it's actually airworthy, at least in California.  That's
completely different from sales or use taxes.

Skip Slater