So I can fix with glue once I've used the pva and plastered the window sill?Sure you can - as the widths are usually more than the average plank of timber, MDF is an option and its fine so long as it is painted / protected or whatever.
25mm (1") is a typical thickness and its dead flat. Fixing with No Nails or similar adhesive is fine so long as the timber has plenty of contact to the adhesive.
John