The Surface Pro 2 supports an external monitor. For touch screen support, you'll need to use the USB cable to attach the monitor (touch enabled of course) to the computer.
You have different offerings available (no experience with them).
You have an adjustable monitor: the Dell S2340T, it's also a doc station with speakers. This is a fancy one
You have a less expensive, but is just a touch screen monitor, and does not feature a such adjustment when using the monitor (just tilt, but can be put near flat), it's the S2340T, there is a 27inch version S2740T available.
The monitors mentioned above, all have a 1920x1080 resolution, all features an IPS panel (the same type of panel technology found in your Surface Pro 2).
Touch screen aspect and the fact that it is an IPS panel, contribute to the higher price tag of such monitor.
I strongly recommend to always get an IPS panel monitor. They cost more, but as you can see from the Surface line, colors are better, the image is a bit more sharp over cheaper technology offering (TN) of the same size and resolution, and view angles allows you to perfectly see the screen no mater how you sit. Also, as IPS panels are targeted at high-end consumer grade monitor solution, and up (professional grade), you have a nice strong build quality and great features.