Hi Ian,

Welcome to the forum!

As always, the first bit of advice for new builders is "DON'T BUY ANYTHING YET!" - the temptation is strong, but to get a good design you need that flexibility. Otherwise you'll hamper yourself immediately if you're intent on using pre-purchased components, leading to compromise after compromise.

You've already done a reasonable job at the next step, which is setting out the requirements of what to cut and how accurately. Next, focus on the mechanical design. Try to CAD it out if possible, it will give everyone a much better idea of what you're planning.

The first thing I've noticed is that you say you want to cut 800x800, and then you've specified linear rails and screws that are 800mm long. You'll need these to be longer than the actual cuttable area, because bearing blocks and ball nuts take some space. You also don't want to be hitting the bearings or endstops in your cutting area, so you need some margin. Plus, the gantry will have some width, which will reduce the cutting dimensions.

Looking forward to seeing some designs and questions soon :)