I have had cats all my life, usually rescuing adults from the local SPCA, but we've even adopted a kitten from time to time.
I think your kitty would like a companion. Sasha, a female I have owned for about 10 years by now, shows very close to those same symptoms if a companion of hers gets lost. She would sit staring out the window for hours, and would mope around the house and yard. She would also at times get very clingy, you would pick her up and pet her for a time, but when you went to put her down she would positively scream, and hang on for dear life.
It does not matter too much what age the cat is at, though I would suggest that unless it is a very calm youngster, or a very energetic older cat, to try to keep their ages within about 3 years of each other. It seems to cut down a little bit on any problems, be it right away or in the far future.
Also, I would say keep the two cats separate for the first few days at least. Make sure they each have their own separate room, with their own facilities, so there is no fighting. Then, after your newcomer has had a chance to settle in, introduce it (supervised) to the older cat, for a short period of time. If things get heated, separate them quicker, but make sure each have had a chance to see the other, and to smell them.Then call it a day and separate them again. After that, every day(unless the cats are especially shaken up by the encounter, then it may take longer for them to settle and relax) have a short, supervised meeting between the cats. Eventually, you wont have to supervise as closely, and be able to leave them together for longer and longer periods of time.
This way may take a longer time that you would like, but in the end the cats will at the very least be able to stand the sight of each other. After they are comfortable around each other, you can move both living quarters together. Remember that this is an especially stressing time for both cats, and give both lots of love and friendship, and make sure they are both healthy and are not losing weight or having troubles going to the bathroom.