More dumb questions

So I am getting used to my B reg late series 3 but have a few dumb questions:

I have standard wheels but unsure of a good make of replacement tyre. Just had a puncture and the tyre wall is knackered. What make and size is standard?

Second question: The landy has lots of gear knobs. From what I understand the red one is high ratio if pushed forward (for normal road driving) pull back for low range for offroad or towing. What does the middle neutral position do?

The yellow know is basically push down for 4 wheel drive?

I also have an extra black knob with a big "P" on it, what is this for?

Last question... The steering has lots of play. It just passed its MOT but I want to tighten it up. Is it just a case of adjusting the steering box?

Thanks

As you're vehicle is over 30 years old it probably hasn't got it's original size tyres on it and unless you are replacing all four you should stick to what ever size the other three are. As for brand what are you mainly using the vehicle for and what's your budget?
Black knob could be an overdrive?

As for vague steering it could also be worn track rod ends.

Welcome to the forum and get some pictures up to help with identification