Hi Marian,
As I messaged you (privately, and at length) I spent the entire last summer in Fiordland. As I shared, 80%+ DEET is THE way to go.
Put that on daily, and sandflies aren't much of an issue at all. Even at Sandfly Point at the end of the Milford Track, or any of the other truly heinous sandfly hangouts in Fiordland.
As I also offered, I have a nice little collection of topographical maps, of which you are welcome to borrow. Furthermore, if you are going to places that I don't have a map for, I'm happy to buy it/them and lend them to you, to help keep your costs down, not having to pay for something you're only going to use once. Eventually I want to collect all of NZs topo maps, so it's no drama for me to do this.
Whilst you are not from New Zealand, despite what some claim, New Zealand conditions are NOT more extreme than what you seem to have experienced at length, and traveled to higher altitudes than most people on here, I'd imagine. As you know, wherever there are mountains, there are micro weather systems/patterns. Plan accordingly, and Bob's your Aunty.
So don't be put off from working out the exact trip YOU want to do, then doing it. Yes of course, there are some extra considerations when tramping alone, but many of us on here do it all the time, despite some of those very same people vehemently warning against it.
Also, the chances are good that you're going to meet people here organically that will be more than willing to join you on a tramping trip, whether they be locals or - more likely - other travelers.
So long as you know within yourself that YOU are prepared, you'll be fine.
And yes, a PLB is a must.
I suggest that you don't buy one unless you plan on using it extensively all over the island/country.
People are often quite reticent to buy secondhand PLBs; it's just not often done. So you could find yourself stuck with it or having to practically give it away. Not to mention the logistics of having a local address/emergency contact to register your PLB with.
You can always hire them for short trips. Or, depending on when you're traveling, and how it marries up with my school term, if I'm busy and unable to get out tramping myself (I'm taking on 165 credits next year), you could potentially borrow mine.
Don't stress too much; plan well, and you'll have an adventure of a lifetime! :)