InterLink DX will be the easiest one to set up with RealFlight, because you just plug it in and the controls are all set up for it the way RealFlight expects. Every switch, button, lever, and knob is programmed to do something with RealFlight, which in a way makes it equivalent to about a 20 channel radio.
For a field radio, I'd never suggest anything less than eight channels. In Spektrum's lineup, the best value right now for a new radio is the NX8. For the small cost difference from NX6 to NX8, the NX8 is well worth it. NX7e is a bit cheaper than NX6 but has quite a few features removed to try to lower the cost. Horizon's BNF planes and helicopters are configured to work with six channel as much as possible, but they create some limitations when they do that. Six channels was "the norm for beginners" for a long time, but now that receivers have programmable features such as flight modes and gyro tuning, six isn't enough to really enjoy what you're doing without unnecessary hassle.
Six channels will be really limiting with RealFlight, too. The USB wireless receiver dongles are limited to eight channels, so if you want to go wireless, you can't use more than eight. All NX radios can be used with RealFlight via USB, and you can use all 10 or 20 channels of an NX10 or NX20 that way.
