Complete Website Upgrade Needed

I need my 2 identical websites at [login to view URL] & [login to view URL] to be upgraded in a number of areas. I require the current account login section to be upgraded and allow specific users to access a reserved section of the site, using their chosen username and password (currently email and password that I have no control over as it's automatically generated by PHP script). Once they are logged in I would like them to be able to set a variable which would allow them to access all products at various discounts and purchase and pay for them at this reduced price. I would also be interested to hear from anyone who feels they can offer a completely upgraded site and be in a position to provide me with an example [login to view URL] etc. I am anxious to retain my hard thought SE rankings also so would need any work/new site to incorporate quality SEO practices to maintain or improve my current 4000 unique visitors a month. If anyone has any ideas with proven track record/past examples etc then please feel free to bid. Kind Regards Nick