Fortunately, inexpensive 20"–24" monitors that will work are readily available. 22"–23" seems to give the best bang for the buck.
Those that are backlit LCD are brighter (like your iMac) than those that aren't. Don't worry that the connections are HDMI and VGA as inexpensive adapters are readily available. If you can get one with a cable, you'll save money over buying it separately. The Apple adapters are expensive and unnecessary—the generic ones work just fine.