If you are going the w/d route, getting one with a built in skimmer is a good idea, it will save you a little work in getting things setup. With 100 lbs of rock in your tank, you have a pretty good bio filter going without the w/d at all. And as stated above, the w/d's can cause some nitrate problems if you don't stay on top of the maintenance.
Depending on where you get the rock or rubble, you may have some die-off once you get it in your tank. This can cause a mini cycle, especially if you pull all your bioballs at once. You probably have sufficient rock now to absorb the spike, but you would be better off to recure whatever rock you get, and swap the balls/rubble a section at a time. The easiest way I can think of is to start removing your bioballs now so that the full load of bio filtration is transferred to your rock over time, rather than all at once. Maybe pull 1/4 of the balls out each week for the next month. That will also give you enough time to cure your rock/rubble, and it will be ready to put in your tank.
If you go this route, you won't need to buy a new sump or w/d, just use the housing you have now. That will save you a lot of money.
As for where to get your rock/rubble. The cheapest method would be to buy plain base rock from your lfs
. Besides rinsing it off, it won't need cured and will become part of your filtration in a couple days. If you want rubble, check garf.org and look for their "garf grunge". It's all aquacultured and from what I've read, a large mix from multiple tanks.
Also as for your nitrate problem, what kind of substrate are you using, and how deep is it?
If the 600gph # is true (did you factor in head height?) You are turning your tank over about 6 times an hour. Try to raise that to 10-15 times an hour and your filtration will be more efficient.