No worries :)
The earliest known serial numbers for Carreras, at least that I'm aware of, are in the 53xxx - 54xxx range. Anything below 60xxx is very early and would carry a considerable price premium over a later-production example; Carreras below around 65xxx are a little less early and a little less valuable, but still more so than one with a later serial.

A note - if the watch turns out to have a serial number in the 53-59xxx range or so, the movement should probably be marked Ed. Heuer & Co rather than Heuer-Leonidas. Some early Carreras have a hex-back case, though I don't think all had them. The seller should be willing to accommodate questions and requests for photos, of course.