I did a quick search and saw this post on the Level1 Tech forums: https://forum.level1techs.com/t/multiple-monitors-in-virtualbox/115445/3 and it seems that there is a driver issue (if that's even a thing with mac, never used one tbh). They mention a "VirtualBox tools" package needs to be installed on the VM (MacOS in your case).
EDIT: Really wish they embedded it into the software tbh!
£280, gives you 24 cores, 16Gb RAM, Raid card & a 1TB HDD to start.
With the remaining money, pick up an ATX case (1U chassis are bloody noisey) a couple of fans, and a couple of cheap WD Green drives.
I've made several similar, all I can say is, they work well, can be good to game on (with an overclock), great for rendering, and great for virtualisation. Do your research, and talk to others that have experience and you'll be fine!
Could you provide some more information? It this going to just be for files, or would you want it to be able to handle something like Plex and be able to transcode video too? Also, does that £350 budget include drives?