Is there space for a separate WiFi card? I'd love to upgrade from my current case to a P-ATX, but my motherboard doesn't have build-in WiFi and I don't see any renders or pictures with a slot or space for a separate WiFi card...
You will not be able to fit a PCIe card, no. Your options for Wifi is as follows:
Assuming you own an ATX board (thus limiting yourself to 2-slot graphics card), you can get a M.2 PCIe Wifi card and some pigtail RP-SMA adapter (Riba thoughtfully included holes at the back of the case for antenna ports to screw through), or use an USB Wifi card (which in most cases is more than good enough. Some USB wifi cards will even let you hook up external antennas to improve signal pickup pattern)
Assuming you own an ITX board (thus are able to use the full 3-slot space, you can either get an aftermarket bifurcation riser and use a 2-slot graphics card with a PCIe wifi card. Doing this cuts the bandwidth available to your graphics cards in half, and require the motherboard to support bifurcation. Or you could still use an M.2 Wifi card (will take up a free M.2 slot in your motherboard) or again, use an USB wifi adapter.
For reference: M.2 PCIe Wifi card:
https://www.scan.co.uk/products/int...211b-80211a-80211g-80211n-80211ac-bluetooth-4
USB Wifi dongle:
Both options above are pretty affordable. PCIe bifurcation riser is probably gonna be more expensive and more hassle than it's worth.
In either case, it's probably gonna be a lot less hassle to just buy an USB wifi dongle.