Greetings,
I have read initial high level requirements & as far as WordPress platform is concerned, why not, as WordPress is VERY user/SEO friendly, in fact, our team has done some extensive work using WordPress framework, so I do not think there is any issues using WordPress. However, the difference of using WordPress or CodeIgnitor/Yii etc. is that without WordPress, you have to do everything from scratch and for such bigger projects, it would be a big project, hence it will cost you a lot of money but this way you will have everything well defined and more scalable than using Wordpress.
Let me know, if you are willing to discuss further and move forward, we can then surely schedule a meeting.
Look forward to hearing from you and talking to you further thru' Freelancer message board.
Regards,
YK
LEADconcept