That's not to big of a window to affect he ultimate package and hardware available. So it boils down to what exactly do you want from this machine? Gaming? Work-desk/spreadsheet? Video editing? Internet browser? Will a monitor need to be sourced? Will an operating system need to be sourced? Is there anything you have already you can re-use?
Reason being on the last 3 points it may make the difference between suggesting a full package from someone like Dell or HP, as parts will possibly exceed your costs, and starting with a P280 case isn't helping your cost flow for parts/requirements.
Might be best to list your needs and expectations, as you may find this project adds up to a little more than you expected. -- The only thing necessary for the triumph of evil is for good men to do nothing - Edmund Burke