It looks like theres a lot of species of oak down here. Would that be the most recommended or are there kinds I shouldn't use? Also will these sink or do I have to waterlog them like everything else?
I think any species of oak will be ok. It depends on the individual piece as to whether or not you'll need to waterlog it before it will sink. Boiling will speed up the process if you have a big enough pot.
As for the sand, I wouldn't mix the black and white together if you want a natural look. Look for something more natural like play sand or river sand (beach sand will have high shell content and might increase the mineral content of the water, not ideal for SA fish). Check out the builders sand at your local hardware store, I guess it varies depending on where you live, but the stuff we get here is $6 a 25kg bag and perfect for a natural looking substrate.