Don't count on the soak. I just got three kegs on ebay that are dirty. Dried syrup takes scrubbing to get off. I soaked one keg in hot Oxy overnight and it didn't remove the syrup. You have to scrub the inside at least twice. If you can't reach the bottom with the scrub pad, use a broomstick. Get ALL of the syrup and black grime off or your brew will taste funny.
Also, if you have a single handle keg with a srew on handle, make sure you unscrew the handle and clean that too. I forgot about the handle on my 3 gallon keg and it was leaking CO2 after I filled it. That sucked. I transferred to a 5 gallon and dismantled the 3 gal handle and scrubbed it.