Wordpress or Expression Engine for my CMS solution?

Hi, I am web designer (front end) who is a big fan of WordPress but quite new to it and don't really understand how far it can extend regarding functionality. I am well aware of the millions of plugins but not being a developer myself, do not really understand the technology behind the gloss.

However I have just been commissioned by a a quite large client to redesign their website and undecided if WP or Expression Engine would be the best route for the CMS and required functionality.

Their main requirements includes

1) A slick designed website using parallax movement and is responsive

2) Multi-language versions of the site for 3 languages

3) A secure membership / customer service area to handle customer requests including return request for their B2B products however they don't sell online.

4) New Account Registrations - which need approval and require acceptance of their terms

5) B2B Accounts are online - to manage all their clients details, queries online

6) Future connection to Sage Accounting to relay information between their account package and the website.

From the CMS point of view WP can probably handle everything and I have found a great theme to start with and push on, the design is done, so my main concern is WP robust and expandable enough to work well for large company hoping to really integrate online into their business. If I go down the EE route I will be paying a lot for a freelance developer to do most the work and totally reliant on them and is EE overkill for a small corporate content site.

Also is it easy to talk to other applications such as Sage.

Thanks in advance for any advice, please feel free to ask any questions.