You've done your homework.
Make sure the router you choose will function with "wifi as wan" - I don't think you can connect the MiFi to the router any other way. I use a Cradlepoint MBR-95.
There are several variations on the Wilson Trucker antenna, any one of which will do fine. Biggest challenge is that with the advent of 4G service, it gets harder to design an antenna with broad enough frequency response to handle both 4G and 3G well. The Trucker series has about the best gain in the 4G range of any non-directional I've found.
Can't speak for this particular mifi device.
4gb isn't a whole lot of data. Might want to look at the Millenicom offerings. Their "hotspot" service uses the Verizon network, and you get 20gb a month for $70. The hotspot is $135, and there's no contract - it's a month-to-month service.
al