I'm not qualified to rule on what level of dress a mess dress jacket would fall under. Nor did I stay at a Holiday Inn last night.

Why buy a shirt and tie (if appropriate), if you can rent on short notice? If renting, you may want to consider a black vest as well. I seem to recall that mess dress jackets are cut similar to tail coats, just without the long tails. Substitute some silver-tone buttons (from the local yardage store), and you have yourself something that approaches a Prince Charlie, without actually being one. But it does sound like this would be classed as a 'Black Tie' level outfit, if paired with the horsehair sporran.