One problem i see with the Eton style jacket is that the collar is shawl style (rounded), while all the scottish jackets I've seen are notched lapels (the cut-out triangle). If I wanted something really formal for an important event, I would rent or buy the authentic pieces, or not go. If I didn't have a lot of cash, but just wanted to look a little smart, I would probably do what Richard did, and have a regular jacket tailored. I would probably pick a solid color other than black, since I wouldn't be using it for formal.