Should we book a guided tour in Iran?

A guided tour is the only practical option in Iran. Although English is quite widely understood, you need a guide who speaks Farsi. There are also various protocols that apply to foreigners and without a guide to advise you it would be easy to transgress. Just organising transport and hotels becomes problematic. The clincher, however, is that one of the requirements for a visa to Iran is a reserved itinerary, and that's just too hard to arrange yourself. 

Several Australia-based adventure tour operators offer trips to Iran including World Expeditions , Peregrine Adventures  and Intrepid Travel . Rather than one of these small group tours you could book a personal tour with an Iran-based operator. One possibility is Uppersia , which does customised tours for just two, with a driver and an English-speaking guide. Their reviews on Tripadvisor are excellent and tours are available at different price points, depending on the style of accommodation. 

The best time to visit is the cooler months from mid-March to mid-May and October to the end of November. Note that the US Congress recently passed legislation that affects visitors to Iran. If you have visited Iran and you subsequently visit the US you will not be entitled to apply for admission under the Visa Waiver Program that normally applies to Australian citizens. In that case you will have to jump through a few more hoops, including attending an interview at a US consulate or embassy and paying a fee.