It’s super useful to have internet on your phone while travelling, but roaming plans can be very expensive. There is a better way! Download a virtual Italian SIM card (eSIM) before travelling, or when you arrive at the airport (all airports have free wifi). I use airalo (www.airalo.com) every time I travel, they have e.g. 3 GB of data valid for 1 month for $10. You can use this for internet and still receive messages & calls on your normal SIM card at the same time. Awesome. Just google your model of phone first to check if it accepts eSIMs, since it’s a fairly new technology.
You can drive all over Europe with a NZ driver’s licence if you’re comfortable driving on the ‘wrong’ side of the road. Before leaving NZ, you should get an international driving permit from the AA (https://www.nzta.govt.nz/driver-licences/kiwis-overseas/). If you want to rent a car, you will need a licence and a credit card (not debit card, even if the terrible budget rental place in Gatwick lets you book and pay first with a debit card 😐).
Lots of Milanese speak excellent English, especially in the hospitality industry. This becomes less true as you go into smaller towns in Italy. Luckily, Italians are incredibly patient and will always find a way to communicate, so it’s never a big problem. Download Google Translate onto your phone, and have my phone number on hand in case you get stuck.
Almost certainly not, Kiwis can visit most countries in Europe for up to 3 months as a tourist with no visa. To be sure, there is more information here: https://www.safetravel.govt.nz/travel-tips-travel-europe
Very safe! Keep an eye on your bags when in train stations (Cristina had her phone lifted out of her bag when changing busses in Barcelona a few years ago), other than that I don’t act any differently than when visiting Wellington or Auckland. Just don’t forget to look the other way when crossing the street :)