Assembling a batch of nice fresh parts is a lot easier than you think. Unless you have to assemble/rebuild the whole engine by hand, you will probably spend more time waiting for the paint to dry than you will putting it all together.
Not necessarily; ever put together one of those Saga electric guitar kits? I've spent maybe 5-10% (out of maybe 15-20 total build time) tweaking and adjusting, finding they didn't drill all the holes in the right places, etc.