Fill the room with candles. Beg and borrow every tall candle stick you can and put 3 or more on each table, have them lighted when guests arrive.
And, the way the buffet is laid out will add a lot to the ambiance. I would recommend lots of mirrors, puddled cloths, different levels made with boxes, very tall candles and a couple of arrangements made in those yard long vases.
Linda C

One of my sisters wants to use dark purple flowers - any thoughts on what type of flowers to use or interesting ways to arrange them?

Balloons shaped like prunes.

You can get nice, full 10" potted mums in dark purple. Place plastic pots inside baskets that have been sprayed with gold paint. Tie a few helium-filled gold balloons to the basket to float overhead (about 36" over the top so they won't hinder eye contact across the table).

Get extra flowers for the buffet table and the front door.