I have built and use an MDF router, although with a larger cutting area (around 900x400). I use skate bearings on 2" steel tube for X and 1.5" tube for Y. I would try to avoid using the skate bearing approach as they are a fiddle to build and set up, and probably a bit big and clumsy for this size machine anyway. Supported rail and linear bearings would be much easier to install and set up. I've seen working routers using this approach and although it is not as good as profiled rails, it's probably a good engineering compromise for this purpose. I would go up a couple of sizes from your "8mm" comment above, though - my 3D printer uses these and they are only just enough for a machine that hardly generates any cutting forces!