You'd pretty much be dressed and equipped like any other European fighting man, though you could add a tartan cloak (NOT a belted plaid or "great kilt"). A majority of Scots at that date were still Gaelic/Celtic, with some Anglo-Saxon incursion in the southeast Lowlands, and with of course an influx of Norman nobility. So, your Scots man-at-arms (if Gaelic and not Norman), could very well wear a bit of tartan, a cloak or mantle being the most likely item. The English mocked such brightly colored garments as being "Irish", i.e. Gaelic....