I don’t really know much about mobile networks, but before I found this post I did some research to find out if either of the modems would work okay in Australia. If I understand correctly, it looks like the European modem would cover all of Vodafone and Optus UMTS and most of both their LTE bands but unfortunately missing the most common low LTE band, though I assume 4G will work fine in most places with these networks. Both the SIM7100A and SIM7100E modems are missing important bands for Telstra. The SIM7100A supports only the mandatory 850MHz B5 for UMTS and nothing else. The SIM7100E supports most of the other bands except the mandatory one. I decided to search the forums here to see if anyone had mentioned about compatibility with Telstra and found this post.
I personally would really like a phone that can also be used on other networks and overseas.
Same for me, I would be willing to pay a bit more and/or wait a bit longer if necessary for that to happen. I understand that it may not be possible within the constraints. For anyone who is wondering about it, there is a development post that talks about the limited choice of modems here.
@BarryK, have you managed to get in contact with anyone from Purism?
Edit: I tried to make my post more clear. I was meaning that I assume Optus/Vodaphone will be fine, Telstra was my concern as with the OP.