The navigation computer display, more properly called a CDU for computer display unit, or MCDU for multi-function computer display unit, will offer a real challenge. It is typically 5.75" wide and about 8.75" long. Into this package are stuffed a 5" display (older units are monochromatic, newer are color), a numeric keypad, an alpha-keyboard with a totally non-QWERTY layout, a handful of function keys, two rows of context dependent function keys, two enunciator lights, and a brightness knob for the display. And of course these units have provisions for lighting the panel (technically called a "light plate") for use in low light conditions. Most airliners have at least two or three of these beauties.
If you're really lucky, you can pick up a surplus CDU. (Try Ebay and Trade-A-Plane.) These units are not flight computers themselves. They act as the display and keyboard for a computer, and there is the possibility of interfacing it to your computer. You don't actually have to be a rocket scientist to figure out how, but I suspect it would help. Probably it's not that bad, but the (very) few people who have done so have strong technical backgrounds.
Simulated CDUs developed for training purposes are available. (Check out Command Fliteware, International.) They don't necessarily have the look and feel of a real CDU, but they do have the functionality and are suitable for procedural training.
Keyboard overlays (covers, skins, etc.) offer another possibility. This won't give you the display, but you will have a better method for input than using the mouse. A company called Cockpit Collections used to have an assortment, but their web site (http://www.cockpitcollections.com) has gone 404.
A totally software approach is to purchase some of the Project Magenta software. This draws a very realistic image of a CDU (not to mention other panels) on your computer display and backs it up with quite realistic CDU functionality. And by all means check out Robert Prather's shareware (?) "ProMFD" panel software.
There's also the do-it-yourself approach. Keyboard switches (called "tactile pushbuttons") are available from any number of electronic supply houses. For instance, try Jameco Electronics, Digi-Key or Mouser. Key tops, both round and square, are also available for these switches. You can mount the switches on perf-board (also available from Jameco, Digi-Key, Mouser, etc.), a circuit board material with holes every tenth of an inch. Interfacing these switches to your PC is as cheap as a replacement keyboard. Pick up a keyboard (hopefully on sale or at a swap meet) and wire your switches in place of the keyboard switches. There is a pretty good article by Robert Prather on this sort of thing at FlightSim.com. Go to the "How To" section and look for an article titled "Keyboard Emulator".
The CDU incorporates a small monitor, typically a 5" diagonal unit. Older CDU models had monochrome displays. Newer models use color displays. Five-inch color VGA displays are occasionally available to the hobbyist. Precision Flight Controls carried a CRT model a while ago and perhaps still does. Color LCDs have their appeal as well. Seems like everyone makes them. The issue is finding a company that will sell in small quantities for a reasonable price. What you're looking for is a display module with a VGA interface. Buying the LCD display alone is not particularly useful as the interface is pretty complex. You really want someone else to deal with the interfacing, so look for the complete display module.
Five-inch LCD display modules with NTSC television interfaces are increasingly becoming available. Among the usual electronics parts suppliers, you might specifically check out Jameco Electronics, Parts Express Electronics, Apollo Displays, EarthLCD or EIO. These units are not terribly expensive (US$100~$150) but the fact that they are designed for television may be something of an issue. Beyond the fact that their use will require a video card with an NTSC output, five-inch NTSC units are made in different resolutions, and even the higher resolution displays aren't equivalent to VGA displays and may not have adequate resolution for your application. They certainly won't be adequate for normal desktop system fonts. However, if you are using large block letters and very simple graphics, you may find these displays to be a worthwhile addition to your sim.
Interestingly, black and white television monitors may well have adequate resolution at a substantially cheaper price. If you check out the electronics section at your local discount chain store you're likely to see a very inexpensive, 5" B&W portable television. I have seen these little imports on sale for as little as US$18. (Naturally, this was right after I paid US$25 somewhere else.) By today's standards, I judge them to be pretty wretched televisions, but they have two redeeming properties. They are really, really cheap, and some have an A/V input. The A/V input means you can pipe video in directly without having to hack the circuitry. Going through an RF modulator and the wretched TV RF section places severe limitations on the video bandwidth, killing your horizontal display resolution. With an A/V input, these units have promise and just might work as a monochrome display in a simulated CDU (or HUD).