Used equipment can be a good bargain, if you know that it is in good condition or buy it from a vendor who guarantees it. Otherwise, it is a gamble. The times I have bought used equipment without a guarantee--only twice--it has been fine. However, you can't be certain.
Those two cameras are similar in design and controls. The 550D is a newer model. It has a much higher resolution sensor, and it has automatic sensor cleaning. I don't think the 450 does. However, when I switched to digital, I used an earlier version of the 450, and I got some very good photos from it. This site might be helpful:
http://kiyas.la/en/digital-camera/compare/Canon-EOS-450D-vs-Canon-EOS-550D