Everyone has given you good advice.
Here are some extra pointers though...
1. Consider what your reason for purchasing a REO or SS home is? Sometimes better deals can be found by motivated sellers!!
2. Interview a few agents, and find one that you know will look out for your best interest. If their only concern is if you are pre-approved for a loan, be concerned!!
3. How much down... It will mainly depend on your method of financing.
Hope this helps..