Exterior door jamb construction
whenever i build a custom exterior jamb, i use ipe.  its stronger and harder than anything else you will find for exterior work, takes paint well, clears beautifully, and is a great wood in general.  the biggest problem you will run into with exterior jambs is moisture related warpage. ipe wont warp. 

to be honest, i learned long ago the best way to do what you are doing is just build the entire jamb from scratch.  that way you get all the stops, seal mounts and extensions as an integral piece of wood.  you can make your own threshold from oak or ipe and its never going to fail.  if you do want an aluminum one, try here:

http://www.crlaurence.ca/crlapps/showline/offerpage.aspx?ProductID=28847&amp;GroupID=58652&amp;History=39325:4939:26565:28809:47252:28809&amp;ModelID=58652&amp;pom=0#

you will have to mill a sub-threshold out of wood and joint the aluminum one into it to get it work as you wish

if you try to cobble something out of an off the shelf unit, you will have caulking and glue that will eventually let in water, or you have to set the door so far towards the outside, that you get hinge swing issues with your inner nailer extensions.  

you can definitely buy an exterior door, and all the seals.  then build your own jamb and put it all together.  you will be much happier for much longer.