Newbie ordering kit starting parts collection for full MKIV IRS.

[Neuxstone]

Just heading into wild blue retirement yonder!!!
The wife loves the thought of keeping me busy so she GREEN LIGHTED the MKIV build late winter.
I did a search but got muddled up with generic information.
Question (first of many in the next few months)(followed by many more later I'm sure)
is...: I'm looking for a good used IRS. What one should I be looking for? Where should I be looking? And like everything else is going to be in this roadster; should I get a new/rebuilt.
Id like to stick with Ford products.
Does the T-bird or 95+ Mustang sport these? I've seen the Lincoln MKVIII suggested...help please if you have any info.
Thanks. Dave.