Do I need a Public Certificate or just public DNS/IP

Hi,

just wondering if any one can help me out.

Current Environment:

2 Universal servers sitting as a cluster in DMZ with NAT address to internal corp network. This is a purely WDE installation and no encrypted email consumers.

Problem

I need to allow external (home based) WDE machines to update over the internet to a pair of Universal Servers sitting in the DMZ.  I do nto have the luxury of a VPN.

Do I need to obtain just an external IP address and DNS that points to a NAT which will be configured for both servers or would I need to obtain an SSL cert?

Hope someone can help.

Cheers!