Colways MT's are cheap and good off road, but noisy on the road and wear quickly. Their AT's are also good. Suzuki ran 215/75x15's (if my memory serves) on their demo fleet, but can't remember the make. Any bigger and you'll need to start modding bodywork etc.
As for steering guards - I've not needed one so far, if your jimny is still standard then the plastic bumper amy be prone to damage but gentle laning /playing shouldn't cause problems in the steering area.
