I think I would go with a sealed wheel bearing unit from a 1ton or larger truck FRONT end..I see you are in the UK so models will be different but one of those trucks have very heavy bearings usually rated for around 12,000lbs (5500KG) vehicle weight..they come with flanges with bolt holes to mount it to what would be your lower table and the wheel studs then pointing upwards would mount your excavator house.FWIW most that are 4WD then may have a hole in the center that will allow a pass-thru for a hose or two for drive motor or auxiliary..hope that helps