Of cos, you need to purchase a proper portfolio case. No need to be expensive type. So long can hold your artwork can le. If your artwork has a size up to abt A2, then get a portfolio case slightly bigger than A2 one. Bras Basah Complex is a good start to look into that.
Preferably, mount your artwork on a piece of black mounting board. If possible, use a consistent size for all artwork mounting. If largest piece is abt A2, use black mounting boards thruout. Looks more tidy when place in ya portfolio case. If you wan to be careful not to spoil the artwork after mounting it... overlay it with a piece of mahjong paper.
Can't help you on the interview part cos me from NAFA previously. However, we were required to draw still life during the day of test. Can't rem if there's interview or not... too long ago liao.

Good luck.