Yes, there's an external antenna connection (which you'd need a patch cable for).
Yes, you can charge it via any external USB power source (mains, car, computer, etc).
The recharge I use is 10GB for $150 which has 365 days expiry (a full year).
Also, any remaining data allowance is carried over if you rechage before the expiry date. (Eg: if you recharge at 364 days and you have 5GB left, you will have 15GB for the next 365 days.)
I don't have one of these wifi modems, but I do have Telstra 4G prepaid modem which uses the same rechargable sim.
You will only get 4G speeds where 4G signal is available, which at this time, is most metro and major regional cities CBD's and some metro suburbs.
In other areas, it will operate on NextG / 3G.
If it's intended just for home use, it will cost you more to run compared to what you are paying now.
Eg: you currently get 4GB per month for $44. The 4G Elite recharge of 10GB is equivalent to 2.5 months of your current data allowance. So, 2.5 x $44 = $110 compared to $150 for the same amount of data on the Elite.... but, you'll benefit from the 'anytime' usage.. no on/off peak conditions and faster speed... even on NextG/3G.
If you currently don't use all of your 4GB data allowance per month, the Elite may be a better option in that regard, as you don't forfeit any unused data in a month (only if you don't recharge before the 365 day expiry).
The thing most people find is having access to higher speed, they tend to surf & download more than they used to, so their data usage increases.
Hope this helps.
If you plan you also use it mobile, then it becomes more useful.