Buy A Mustang With No Keys ?

A valid title/ownership and then a lock smith and about $150 give or take.

Or here's some information that may offer an alternative on a DIY PATS programming option.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/

I have also reprogrammed PATS keys using the autoenginuity product. However it's not a cost effect option if all it's used for is this.
https://www.autoenginuity.com/products/scantool/

Other options is a hand held tuner with custom tune (with PATS disabled).

Or tow to dealer.

But the problem with all of the above is that it only takes care of the PATS part of the problem. What do you do about getting a working mechanical key?

The need also for a mechanical key IMO it's + 1 on above. As suggested above, try the mobile locksmith for a one stop solution. Don't expect it to be cheap. But this should allow you to have the correct keys for the doors and ignition cylinder.