I see you're hailing from Ireland, as am I.
I couldn't find a decent price for AMD RX 580's in Ireland, so I ended up at
SCAN in the UK, still cheaper than any shop in Ireland that I could find and, that's including the shipping.
The one I linked to above you could probably slot into the 1 x PCIe x 16 slot you have, to get started at least, the rest, I'm guessing you will need to get decent riser cards for.
My understanding thus far is that the PCIe slots do not need to run at generation 2/3, in fact, some sites I've read suggest that, for mining, you should set the PCIe slots to Gen 1 to avoid potential hassles.
Another thing to bear in mind, from my reading around, is that the AMD cards perform at their optimal when you flash their BIOS's to a modified BIOS, so, with that in mind, I went for the card I linked to in this post, as the
BBT video on this card also had a link to the BIOS he created for this card/memory configuration.