Ditto what low_48 said. In addition, you may want a 13/16th's, or a 21mm if you want to turn PR blanks poured into 3/4" PVC. I bought the metric set (3mm thru 20mm). I wouldn't want to do without them. They are so useful.
EDIT: Just a little more info on the larger collets. PR blanks from PVC are slightly larger than 3/4, so the larger collet will allow you to just chuck up the blank without having to do anything.