I'd say go for the 5'X18' router. Take a look at the Shop bot (correct me if I'm wrong), it shouldn't take anything to make it 3 times the length. For the drive, I would go rack and pinion on one side and on that side make it so the gantry can't skew. Our school has a plasma cutter that's got a 12 foot bed on there that can get within 10 thou easy. When building it a lazer level would probably be nice because chunks of iron 20 feet long can flex quite a bit. Umm can't think of much more right now other than I like big pieces of equipment.
