Buy an Argyl jacket. It is suitable for semi-formal wear with a straight tie and for formal wear with a waistcoat and bowtie. Odds are you will rarely, if ever, need a PC or doublet for piping gigs.

Non-pipers tend to favor the lighter-colored Ancient tartans for formal wear. However, people usually expect to see a piper wearing a colorful tartan. Have you considered the Isle of Skye tartan?

Get an 8-yard knife pleated kilt. Pleating to the line or the sett is up to you. It need not be pleated to the line. Some tartans don't look good when pleated to the line, while others look quite good.