Need help re-configuring a current VMWare environment

Small company. We had about 12 physical servers.  I purchased a Dell R900 with 12 processors and 64GB of RAM and installed ESXi 4 on it.  I have virtuallized 5 of those servers onto this one server at this point.  The problem is, is that I'm using on;y internal disk storage for the datastores, and I'm almost out of space (600GB of HD space, 90GB free).  I would like to also virtualize Exchange and one of our SQL servers in the near future.  What are my options at this point, and what can I do to increase disk space without breaking the bank?  Should I purchase a large (10-12TB) Direct Attached Storage unit, connect it directly to the ESXi host and move the datastore to the DAS?  And what about redundancy?  Again, it's a fine line, because we don't need (or have space) for all these physical servers, however when you start dealing with a SAN, redundancy, etc, it's gets expensive REALLY quick.  I've already purchased vCenter essentials and VEEAM as well.  Thanks for any help or suggestions...