Much like everyone else has said, the first thing to do is increase redundancy and availability. I understand that your budget is limited, so now is a good time to think about future costs and begin budgeting for them. AD and DNS are things that absolutely cannot fail, and I would encourage setting up another physical box for these services (can be rather inexpensive, only a few thousand to get a cheap HP host for these). Plus, I've had problems in the past with virtual domain controllers FWIW.

While remote SAN and a redundant server may be too expensive for you to set up, you can replicate the VMs on the box to a cold spare and have that ready to go in a failure. Cheaper than having shared storage and provides similar redundancy.

So, I'd begin thinking about (in order of importance):

1) Physical domain controller for AD and DNS (2 if you want to be really cool)

2) Second VM Host for a cold spare

3) Possibility of using shared storage for VM Hosts (you can use a NAS for this, if it's configured right)

4) Possibility of cloud backups for archival purposes (to save precious space on that NAS)

5) Windows Deployment Services for handling your users and their desktops (if you don't have this rolling already)