If the most formal event you are likely to go to is a wedding, I would go for a lovat coloured tweed argyll and waistcoat which can be dressed up or down with shirts and ties. For a more stylish tweed you could look at the selection at the Sottish Tartans Museum eg the poacher tweed (I am a bit bias in this as it is my all time favourite tweed)
For a functional sgian dubh I would recomend "the Officer" from Gaelic Themes
which has a full tang blade