One Drive now. An unlimited storage for Office 365 customers.
If you’re going to move to the cloud you may want to think about:
1. What happens if the cloud isn’t available? Your internet connection goes down, Dropbox is out for maintenance, etc.
2. Legal stuff: your files (and your clients’ files) are now in the US. Or Germany. Or Godknowswhere.
These are fair points, but with even my sector (Healthcare) buying into it in a big way now, I think you’ll be OK .. you can run the apps offline so you might be waiting for your mail if a JCB takes out the exchange .. but you will be anyway with a traditional setup.
If I were starting a small business now I’d look no further than Google Docs or Office 365 (and would probably go MS purely down to familiarity and for Lync). Cloud based IPT too, for flexibility.