Patrick is correct above.
With a limited budget and time you might also consider going to GA and studying customer behaviour. Find out what pages customers like, and the stickiness & find out when and at what page the customers leaves the website. There is a retail site in India called Jabong - comparable problem. They have millions of hits a month except there BR was in excess of 90% when it started. Slowly work through it page by page, improving each page where customers drop off, until the conversion rate goes up.
Also by finding out what your customer is up to, by reviewing the pages that they like and do not like - you can identify trends. It may also assist in creating content. I often find customers are drawn to 1 or articles consistently - perhaps expand on those articles. GWT's will also in identifing the pages / keyword process to focus on.
It is a great problem to have lots of traffic - now need to tweak away until you make them a satisfied customer. Platforms like unbound and optimizely may also be of assistance. The good news is and it is gold - you are collecting lots of data - and with data you can make informed decisions on what to focus on in the value chain.
Let me know if need any more clarity on any point.