Yes, fixed gantry is where the table moves. That would be a good solution for the size you aim. I would say make it a standard 600x400mm working area. I see 400mm as the width limit for 1 ballscrew moving the table .
Such a machine with quality components will cost you around 2500-3000euro for all included, meaning a working machine with spindle. That's price of parts. You cut the steel and solder.
Motion controller,supported square linear rails and carriages,ball screws, drives and motors, that are the expensive parts.

With that euro/$ relation may be all will be a bit more though, good that you have the GBP

There are some similar builds around, you should have a look. Also people here will help you a lot. There is no set of plans to follow. But you will need to learn to draw in Sketchup, which is quite easy in fact. The other way is to commission a machine to sb. I dont know of any suitable commercial solution for reasonable money.