Perhaps you can find some free of charge, but I think it may be difficult. Also, DIY means that you WILL make mistakes, so that should be added to the budget.
Here is a link to a current build log:
http://www.mycncuk.com/threads/11266-3-Axis-900x500mm
The machine is about the same size you are planning for. Maybe he is ready to help you out, and maybe you can take a trip to him and have a look... Even if he is not in Cambridge, in your case I think it is worth to take a trip even if it might need a night in a hotel or B&B. He can definitely give you an idea about a realistic budget.