What ever you do, DON'T wash your Southdown. One of the biggest advantages of Southdown is that it has some very fine particles of sand. It is indeed the fact that it has these smaller particles of sand that help make it and ideal substrate for a DSB.
I can't convince you why you should try to save a couple of hundred dollars if you already have your mind made up. My argument all along for Southdown was that I can always use a couple of hundred dollars to put towards better lights, better skimmer, calcium reactor....... But now I have another reason.
The 3 Main arguments against SD are
1) it has been stored with other aggregate
2) it has been carried in trucks that may have carried rubish or stored with fertilizers
3) it has been sprayed with oil.
3) First let me say If it is safe for kids, then its safe for animals.
2)Next let me say If it does have other aggregates in it... So what? Some people use most of the aggregates listed in the SW tanks with no problems.
1)If it is stored or comes in contact with fertilizer who cares. What is fertilizer, Nitrate, phosphate, Things that are used in a cycle anyway.
Now if you really want to spend your money and buy the commercially available aquarium sand so that you don't risk your 4000$, even though MANY people are successfully using SD then that's fine. If you are going to worry about stuff like the sand you better hope that you never buy a coral that Has a pest on it. What if you get RTN after your tank is stocked? what about if your heater breaks and you don't notice it? There are MANY things that can go wrong with a tank even if you spend all the money in the world. With all the people that use SD successfully there is no reason to suspect that it will be the cause of the down fall of your tank. I would be worried about many many many other things first. But in the end it comes down to the fact that its your money, your tank. If for some reason it makes you sleep better to spend more money on your tank than is needed then by all means do what you got to do.