After reading this, I think there some differences in what you are calling a Power supply and what people like me who are coming from the computer world call Desk top PS.
However since I know tons about Wifi 3G cards and even what Mifi stands for
. Let me at least help you with that part.
The Mifi's ability to work really depends on which company you have the service with. Most SIM card based units will have the ability to obtain international data plans, you merely need to call your provider and ask. NOTE:! It will not be cheap! a one months 100Mb of international data plans can cost more a a 5Gig local plan! and IF by chance you go over that 100Mb be ready for a real sticker shock!
Your better route would be to purchase a OZ base Mifi unit and once finished there let it expire, even at the higher rates, you'll have better coverage and overall costs will be lower.
Last time I allowed a unit for international data plans for one of my ex-companies travel PMs they had excellent coverage at pretty good rates in Christchurch.
Hope that part helps.
You might also upload a picture off what you are calling a desk top PS. We might be calling it a an inverter + charger system
Cheers
Bruce S