I have an Emperor 280 and an Emperor 400 on my 75 gallon mbuna tank, or just a bit over 9 times the volume of the tank in gph. IMO, it's a necessity with mbuna tanks to have higher than normal filtration rates. That's it, no power heads, and I have no problems whatsoever.
I couldn't tell ya how many pounds of sand are in my tank. I started with approximately 1 1/2" on the bottom. As for rocks, I just have a variety of river rocks. However, the foundation of the rockwork is clay pots, notched and then siliconed for stability. I like this 'cuz it increases the number of nooks and crannies for the fish to hide in without adding significantly to the weight. The pots also take up less volume than rock does. After I put the pots in place starting along the back, out to just shy of the middle of the tank, I used the rock to cover the pots to create an aquascape.
Don't know 'bout the texas holey rock, but I just get slate from one of the local landscaping companies. I know Lowe's has it as well, but the landscaper was cheaper. It was priced by the ton. Don't remember how many pounds I got, or what the slate itself cost, 'cuz I picked up a fair amount of other rock as well, but the whole bunch was 750 pounds for $35. Maybe a quarter or a little less of that was slate.
As for the heater, well my fishroom is heated independently of the rest of the house, so I don't use heaters and have no experience with that brand. A good general rule of thumb is 5 watts per gallon, though. A 300 watt heater may be a little smaller than recommended, but should be adequate as a minimum for a 75 gallon tank. The temperature in your home actually contributes quite a bit to heater efficiency. If ya like it cool, ya may end up adding a 2nd heater in order to maintain the tank at the temp you want.
Cheers,
WYite