I have had a few 1911 models. Impact is a good reference, but can sometimes be inconsistent. I usually use
www.Budsgunshop.com for actual (new) market values. Also check out KSL's classifieds for used gun prices, but keep in mind that a lot of the sellers on there are opportunists. You can often find new guns for the same price as a used one selling on KSL. You just have to know where to look.
As for a 1911, it is a great platform and can be a great gun. It has a slim grip and frame. GI models are usually pretty basic. No extended beavertail, no extended or ambidextrous safety, spur hammer, basic 3 dot sight, no "speed bump" grip safety, etc.
I have a Springfield Mil Spec full size 1911 and have carried it concealed with out much problem. I'm only 5' 10" and 170 pounds. All you need is a good holster.
I currently carry a "compact" 1911 that has a 3.5" barrel on an officers size frame. No problem hiding it at all. Again, you just have to have a good holster.
Again, if you can get it for a good deal, you won't be disappointed. It is a proven platform that has withstood the test of time.