Man I miss the foundry, it looks like fun.
Here are my thoughts on your project, you may already know this stuff.
The wheelabrator will give you a nice shiny finish, just make sure you strap the part in. If you let it tumble it can get bent and dinged pretty bad, and beware of occasional bb's from being embedded in the surface.
As far as the mold prep, if you use a fine screen for your first layer of sand and with a little higher moister content (the sand inside of the pile of sand your using), you can achieve a pretty good finish. I like using a air nozzle to apply the parting powder as well, just blow it across the bag and it lays down a nice even layer. Clumps of powder leave a positive bump on the surface.
Have fun.