i'm gonna be building a website,for my summer training,but then later commercialize to earn a few bucks
Main thing is, i want to make it as attractive GUI wise as possible with probably PHP only, and ajax and flash are allowed to get as many marks as possible

and it will basically be yahoo messenger or just like IRC chat on a site:
chatrooms, environs, multiple chats etc. and an option of saving conversation to server against the account of the chatter

so does this need a CMS ?

i mean these will be the basic compononents:

first page - just two buttons - start chat, browse chats

start chat -> select a category->select a person->start chatting->end chat with or without saving conversation
now in a chat multiple users can enter - just like in a chatroom or irc channels

browse chats -> search for words -> select any result-> view: just like threads of forum posts like here

So do i need a CMS like Drupal for this?
only things i will need to store will be current chatroom categories' statistics,users' account info and chat conversations of users -> if they want to save them against their account - like an email system of sorts

i've got all the books on drupal and php. but now i'm really confused how to go about developing this site. Drupal is so complex,with all the modules and functions i don't think i would need and reading a few chapters made me think how
much bloated and unnecessary things i've to learn. But then would just PHP code from scratch or with Codeigniter will do it or not in terms of scalabilty ?? I'm confused.

Please help me select the right choice