I have this old plackard that was once available through the Albion Mercenary's Tailor. Would this be a plausible armor choice for a Border Reiver scenario or is it a bit too early for it?

---

 Attachment: 17.24 KB

It depends of which period do you want to use it.
It's roughly based on XV c. miniatures of French-Burgundian chronists shoving lover part of cuirass being white and upper part being covered in cloth and rivets, and surely it's useless without upper part (plastron).
If you are looking for some XVI c. solutions, I would recomend solid one piece cuirass, they are usially cheaper.