I have a 153QE which is basically the same machine but with fewer embroidery stitches. It's a fantastic machine and I read somewhere it tops the list of machines that owners most regretted not keeping when they traded up. I paid $800 for mine two years ago.
My question is...why do all the parts have prices on them? Are you absolutely sure they're asking $900 for the whole package? Make sure they still have all those pieces! Also, you may find that Bernina dealers will no longer add stitches to the 153/163 machines due to their age, plus the software runs on Win 98, nothing newer.