marker pen driven over an 8x4 sheet help.

[Jonathan]

if your only ever going to cut woods with a max thickness of say 2" it needent be a heavy monster

Unless you try cutting it in one pass

If you're only using it for plotting them a timing belt drive (instead of ballscrew) would be fine since the force on the pen is very small compared to cutting. With the stepper kit that Mark just linked to and belt drive it could still be fairly fast as the distance moved per revolution for a belt drive is high ... so the motors don't need to move at a high rpm to get a good feedrate. You just sacrifice resolution which isn't a big issue for a plotter.
I wouldn't advise those motor or that driver for an actual router though - you'll only get fed up with how slow it is and want to upgrade.

I was going to mount my machine on the ceiling originally (so at a funny angle) as that would take up less space and the swarf will just fall away and clear more easily.

now you need to have a look at frame options.... aluminium extrusion, easy to use and a bit pricey.... steel box section needs welding

Doesn't have to be welded, but that's the easiest option. A frame made from wood should be fine for a plotter. Can get away with one of the cheaper bearing options.