Michelle, if you're planning on working around Central the Soho / Mid Levels area might be perfect -- you can walk home most of the time when you're not feeling lazy. And there are a million bars / restaurants within walking distance.
You should be able to find decent accomodation for 6-9K. I would strongly recommend taking a look at
http://www.gohome.com.hk and search around.
Transportation runs all night in Hong Kong, with the exception of the MTR (underground) which stops around mid night. Taxies and buses are extremely safe to take at odd hours.