All you need is a older used laptop that has the following;
• Windows 7
• 1 GB Memory (RAM)
• 1.8 GHz dual-core processor or faster
• Hard drive with 1GB free space
• Internet Explorer 6.0 or newer
• USB 2.0 Port
Look for Win7 and 1.8GHz dual processor (or better) and everything else is likely to be met or exceeded.
For example, here is an old Dell that just meets the min specs for $160. You might want to go with a little more computing power than this, but you get the idea that you don't have to spend $1,000 on a laptop.
DELL WINDOWS 7 DUAL CORE
Also, my reading of the instruction manual says not to use a "power inverter" to power the laptop. I didn't see any mention of not using 120VAC wall power.