Or an e5 series xeon. Lots of memoryslots that can be filled with ddr3 ecc wich is super duper cheap. If you get an engineering sample chip you can even overclock it. (Do get the right one)
its a 2.5 inch drive for laptops. it still connects with sata so just fit it in an ssd mount in your case, they are the same size. if it hasnt rusted inside and does still turn its fine (chance is veeeeery small)
Go with the ssd first, super cheap these days. Get a crucial bx500 series. They are €40 and make your pc so much faster than an hdd. Next id go for the 1080.
in bridge mode from another wifi network of from a data connection?
if you need to bridge it just get a second router and put it in bridge mode.
in the case of a data plan get one of those mobile hotspots.
i think this is one of those cases where its technically possible (probably) but almost no one needs it so it doesnt exist