The offer is for 8GB memory. Try clearing your browser cache/cookies, or use another browser to access the link.
As for the Dell Windows 7 OS disc, no it probably won't activate as Dell doesn't support desktop OSes on their servers. It is a Skylake CPU so it is doable to get Windows 7 up and running --- you'll have to search for compatible drivers. For Win7, always try any available Win Server 2008 R2 drivers as they quite often work with Win7. Win10 is probably more likely to work seamlessly and be able to find drivers automatically as the lower end Dell servers are usually derived from their Precision workstation lines.
In short, buy it with the understanding that it will take some elbow grease on your part to get it running with a desktop OS.