I do not think you can get a similar look using buttercream. If the bride picked out the cake with the fondant ruffles then I think she will be disappointed if you try to do it in buttercream. It will not look the same.
I did one last summer. It was about 3 feet tall. I also supplied additional 'filled puffs' to meet the # of servings needed.
MimiFix gives good advice to make at least one ahead of time to give you a sense of what's involved...
I'd have her set the figures when she arrives.
I'm not sure I'd freeze the cake, but I'd definitely have it chilled in the refrigerator.I guess if you freeze it or not depends on what its frosted with to a degree, Hope that helps!