The HP ProDesk 600 G6 SFF has room for one expansion card

The ProDesk 600 G6 SFF keeps expansion tight. You get one PCIe 3.0 x1 low-profile slot and one PCIe 3.0 x16 low-profile slot — but the x16 slot is shared with the graphics slot, so you can’t have both a GPU and another x16 card.

One notable omission: there’s no M.2 Wi-Fi slot. If you want wireless, you’ll need a PCIe x1 Wi-Fi card or a USB adapter. It’s a weird gap on an otherwise sensible business desktop.