The URA server role poses very few requirements, technically. To be compatible with the operating system itself, you need a 64-bit CPU running at 1.4 GHz or faster and at least 512 MB of memory. 32 GB of disk space are also required, and if your server meets this, you're off to a solid start. It's kind of a "duh", but we'll say it anyway; you need a network card. Beyond that, keep in mind that the server is going to have to do a lot of work accepting connections, encrypting and decrypting traffic, dealing with authentication, and more. Unless this is just a trial-run or lab environment, don't skimp on the hardware. It's not easy calculating the capacity requirements of URA, but in the following sections you can find some more information about capacity planning.
Something that may be of far bigger importance is the placement of the server. We already said you need a network card, and you need to make sure traffic from the Internet can come into the server...