You can absolutely buy data from us. Our contracts are month to month. We do an intiial pro-rata payment (based on the day of the month that you sign up) and thereafter we bill you on the first of every month either by debit order or to your credit card.
If you are planning on LTE speeds, please make sure that you are in an LTE coverage area here.
You can look at purchasing a USB device, but it will depend on whether your router has the ability to accomodate a 3G failover and is compatible with the device we sell. You may want to log a support ticket with the manufacturer to see whether it support this.