Hall of Fame...
Skydive Designer runs best on an iPad
These awesome organisers bought an iPad just so that they could use it!
Let me know if you join their ranks!
If you are thinking of buying an iPad, then the basic iPad (about $300/£300) is a great option. That's what I use, and it should last you many years.
If you're looking for a second hand iPad, then try to get a fairly recent one. Skydive designer requires that you use one of the most recent versions of iOS. Feel free to get in touch if you're unsure.