I'm baffled by how you are breaking down your dimensions. Are you saying that the overall size is 7 feet long, 4 feet wide with the max depth being 2 feet deep? But to account for your shelf area, let's call the max 2 feet deep, but the average 1.5' deep. Does that sound more or less correct?
If so, we're roughly talking 314 gallons--so you're guess at gallonage is just about right. That would mean, to turn over the water once every 1.5 hours, you'd need a pump of about 209 GPH. So, your 500 gallon pump can handle this situation perfectly.
Ok, so your barrel will be above ground. so you do not have gravity in the equation. The reason I asked this is because in the case of a barrel that is at water level there is no chance for overflow because water finds its own natural level. So, let's say something got clogged. In my case my barrel would never overflow because the top of my barrel is a few inches higher than the top of my pond surface. In your case, it could, so the one thing I would add to your barrel is an emergency overflow pipe. This could be a very simple hose attached to the upper region of the barrel (let's say 3" from the top), so that if the water ever went that high, the water would pour back into your pond and not overflow out of the barrel (thus emptying your pond accidentally). Does that make sense to you?
Make that emergency pipe at least 1 1/2" in diameter if you can. 2" would be even better.
2 matala mats are more than enough for your situation. Get the black and the green one as I described in my post. The other colors are way too fine for a barrel in my opinion. On top of the matala mats you can put whatever you want in terms of more bio media. If you're gonna go with balls, ribbon or pieces of PVC pipe just make sure you put all that in a mesh laundry bag so that they are easy to remove. I use the bacti-twist. You would not need more than 1 bag of that if you want to go ahead and buy it. I think I paid $15 bucks for that at thepondoutlet.com, and it's excellent media. It's basically large PVC shavings (make sure to buy the black shavings, not the white). Great thing about it is that you simply give it a shake and all the crud falls right off and you just stuff it right back in your barrel. No fuss, no muss. And it already comes in a bag, so you don't have to buy that.
You can see more info on my barrel at my website,
www.pondmutt.com. The direct link to the barrel making page is
here. I also have links to lots of other barrel/flower pot designs in my DIY section. In some of those other ones, you will see that overflow pipe I mention.
Let me know if any of this was too confusing or didn't make sense.
