1.3.8a 500 error on checkout

[DrByte]

I get a white screen or 500 error. I get no php logs and I've verified PHP logs are working by forcing a basic error.

What are the server's errorlogs saying? What about the server's mod_security logs?
"500" errors are generated by the server, and not by PHP or Zen Cart, and thus PHP and Zen Cart have no knowledge of them happening, and thus can't log them. The details are in the logs generated by the server, and sometimes only the hosting company can view those. Thus, you may need to log a ticket for their help.

I'd hate to have to upgrade as the site was working fine before ...

Before "what"?

and looks like there is a 500 error and other checkout problems for some with 1.3.9f anyways.

And those are almost certainly related to issues other than yours.

Its just been copied over.

What's "been copied over"? From what, to where, with what?

Client really can afford to spend 6 hours on this.

Client should be planning to upgrade to a version that doesn't have known security vulnerabilities.
Have you checked whether all the security patches are applied for the old version you're using?
Have you checked whether there's been any hack attempts? successful or not?