Agree with Rage2.. if you guys are just need a place to store files like invoices, billing, etc.. just use Dropbox. Our small company (>8 employees) have been using dropbox for the past 5 years with no issues. Works out to $130/year/staff. For accounting we use Quickbooks Online.
However I'm starting to look into OneDrive. For $99/year, you get Office 365 subscriptions + 1TB OneDriveper user, so cheaper than Dropbox + access to Office. Do you guys have any experience with One Drive??