es to the previous question. The Helicoil kit includes a drill, a tap and a set of spring inserts which enables you to use the same bolt/stud thread size as before. For the cylinder head, the safest approach is to use a pillar drill to both drill the enlarged hole and to tap the hole afterwards before inserting the Helicoil spring. That way, you make sure everything is machined vertically. That said, I normally do it by hand but you have to be very careful because some of the tapped holes in the head are not very deep and it's easy to end up trying to machine it off axis.
Yes to the second question - it makes no real difference to the process whether the hole is blind or not. However, you need to choose the correct length of Helicoil expressed as multiple and fraction of its diameter, D.