Definately don't go too tall. Typically you can get away with about a 2 inch lift without having to worry about diff drops etc but that said, when I had my previous Navara done, I did end up with an annoying vibration if I left the hubs locked at above about 80Kph.
ARB or a specialist suspension place should be able to sort you out. As for tyres - that is up to you. Bigger tyres will improve the capabilities of your vehicle but they will impact on your fuel economy and acceleration as well.
I wouldn't increase your spring rating if you are only towing occasionally - look at airbags instead. That way you can enjoy a decent ride day to day as well.