--邀請午餐這種禮遇就表明很感興趣
Yes. If they are interested enough they can create a position for you. Another possibility is that they want to know the "inside info" about their competitors.
--我該如何表現才得體呢？有哪些需要注意的呢？
Google "interview meal", "interview lunch", you'll see a bunch of suggetions for what you should wear, what you should order, how you should eat and talk.
--穿麵試的非常正式的白襯衫西裝套裝 Business Suit 還是稍微不那麽正式但也是平日上班裝那種？
Depends on their dress code in company, you'd better a little bit (just a little bit) overdress.
--但對如何表現有毫無頭緒，該多講還是少說？該盡量表現推銷自己還是含蓄兒、不問不答？
Moderate.
