Complete SEO Optimization for a website

We are looking for a SEO professional team to improve our website's ranks and traffic. Four months ago it had about 18000 unique visitors a day but it lost half its traffic. The website is about technology, in spanish and it has about 3250 posts. I'd like to improve it in all the possible aspects: link building, HTML errors and whatever it takes, always White Hat SEO, of course. We're not working in particular keywords, we just want to improve the whole website. We don't want link farms, traffic exchanges, redirections or link exchange programs, only natural, search-engine traffic. We will need an e-mail report every week with the improvements and the changes you've done. I expect an experienced SEO with high rating on freelancer.com who explain what methods will be used for the project and, if possible, a portfolio of what you've done in the past.