Cris G, the only way to avoid data-sampling in Google Analytics without having an access to Premium is day-parting technique = you split a data-request for selected time period into shorter period queries (typically days) and then add all the numbers up. If your profiles/views are not sampled if you look at daily numbers, this could solve you issue.
However, this doesn't work on Unique Visitors, since they will be unique every single time (you are running data requests on daily basis), so there will be most likely duplicates and inflated totals if your site is attracting lots of returning visitors.
To automate some of the work, I suggest using tools like Analytics Canvas. It can make your life much easier and I think it could be the perfect tool for what you need to. Bear in mind the limitations about unique visitors (and some other metrics).
Having said that, I still think the best choice would be to use the benefits of Premium and the ability to get unsampled data for your reports.