We recommend that you use Business Productivity Online Suite for your organization's needs. In addition, we also recommend that you use on-premises server solutions for specific needs that your organization has.
Based on preliminary estimates, you could potentially avoid capital expenses between $10K and $70K. You can get capabilities you indicated by licensing Business Productivity Online Suite for $15/user/month which would represent a monthly operating expense of $375 for 25 employees and $735 for 49 employees. In addition, you will be able to get 99.9% availability for your messaging, collaboration, and conferencing infrastructure from Microsoft.