I have been a Flex user for more than 10 years. Just traded in my 6300 and have a 6400M on the way. I want to take advantage of remote operation and considering an I-pad. I know nothing of I-Pad or Apple products in general. I assume if I buy new, there would be no issues with compatibility, however if I look for a used I-Pad, what models work, or what models would I need to avoid?