I am thinking whether to buy from official Nokia Store or from 3rd party stores. Nokia Store is much safer knowing that it will not try to cheat the consumer. And also I can use 0% installment on my credit card at Nokia Store. However Nokia Store's price will be the most expensive.

Anyone tried using their notebook/computer's wifi to make an ad-hoc connection with their N97 to share their LAN/internet connection?

YES it is useful because Ad-Hoc wifi doesn't require additional hardware purchase (e.g pairing another notebook with another notebook)

I am able to share a direct broadband dialup connection via ad-hoc wifi with another notebook BUT when tried to use the same method to share LAN internet connection via Ad-Hoc wifi, I was unsuccessful. Try to share the internet connection so that the phone can go on to internet via WLAN.