Patio door sill

I'll be replacing my old sliding patio door soon. It is a standard sized patio door and I'm getting the door from an Andersen retailer. My friend and I will remove the old door and replace it with the new door. The new door's RO is approximately 3/4" shorter width-wise than the door there now. I'll need to fit it correctly between the studs with shims, etc. I'd like to purchase all my finishing wood before I start the job but I have this feeling that in order to see what is really behind the edges of the old door's track (i.e. wood condition and size/thickness) I'll need to wait until I pull the old door out.
I am particularly worried about the sill. Are sill widths pretty much the same or again I should wait until I pull out the door track and see exactly the width? I need to replace the sill as it is beginning to rot on the outside edges. The thickness is 1 1/8".
Thanks, Walter
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Don't forget to place pan flashing below the sill to reduce future water infiltration.
No matter how carefully you plan, there is going to be something unforseen. Be prepared to clover the opening with plastic sheeting.
TB
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