While you can use a laptop optical drive(Adapter needed) or hard drive/SSD(addapter needed for IDE. bracket for general 2.5inch drives into 3.5 in bays.) in a desktop, most of the other parts are not made to be moved. Those parts are cheap and can be picked up new anyway.
You are almost certainly better with the DIY desktop route. Some users on here can price you some real nice systems for the price.
You can try to post with this format and i am sure someone will help you get a good build.http://www.tomshardware.com/forum/353572-31-build-upgrade-advice