I think the vertical pipes help airflow. Just like draft forces make hot air rise, gravity makes cold air fall. So bringing in cold air at the top and letting it fall helps things "move" a bit better than only bringing it in at the bottom. Of course you can always add more openings if flow is a problem and as temperatures stabilize and your fire builds up you don't need as much flow either. But if the physics is important to you, I think you want the side vertical pipes.
Now as to PVC or not...as long as you have cold air going down and in, you'll probably keep the PVC below its flow (melt) temperature. So if you have 3 pipes and start with them all open but then valve off two of them, the one you always have lots of cold air flow in could be PVC I suppose. But the other two still have to handle the conducted heat from the fire at the bottom so would have to be metal. Since cosmetically, I think you want all the same, you might as well plan on them all being metal.
But if the PVC question was just due to being on a tight budget, I'd consider galvanized electrical conduit. EMT, the thinnest, is very inexpensive. And I don't believe these side pipes get hot enough to oxidize the zinc and allow it to flake off so I wouldn't worry about health issues but your mileage may vary. (You can always dip the lowest few inches in paint--if the paint doesn't burn off, certainly the galvanization won't.) If you're careful, 3/4" EMT will take a normal tapered NPT die, and although the thread isn't particularly strong, it will certainly hold up the side pipe and not leak at these low inlet pressures. Add a little JB Weld if you want to firm it up even more. And if you move up to intermediate (IMT) conduit, you'll have no issues at all threading it.
Addendum: Another possible benefit of the side pipes is it allows you to put your valves up higher, where things are cooler. You might even be able to use cheap PVC ball valves then. The savings there will easily pay for the side pipes. Or instead of valves, make your own air restrictors from steel plate, soup cans, etc.