The Nexstar ARF is a smarter buy than a Nexstar Select RTF, but I certainly wouldn't try to talk you out of considering the Sig Kadet LT-40 that was mentioned. Any decent .46-sized ball bearing engine from O.S., Thunder Tiger, Magnum, Super Tigre, or Evolution would be a fine choice and would provide plenty of power for your Nexstar and whatever you might choose to fly down the road. The O.S. 55AX, Magnum XLS .52A, or virtually any .70-sized 4-stroke would be a good bet if you want to make sure you have some extra power for float flying down the road.

The Futaba 6EXA 2.4Ghz FASST radio is an excellent choice and a good value for the price. Just make sure you get the four servos that you'll need to fly your trainer, they may not be included with that particular radio. It's the most affordable full range 2.4Ghz radio on the market, and it has plenty of features and enough model memory to satisfy a new sport pilot for quite some time.

Real Flight G3.5 does indeed come with a dedicated controller box that plugs into your PC via a USB port. You can hook a standard radio into the Interlink controller's trainer port so two people can fly on the PC simultaneously, but I honestly have no idea if the 2.4Ghz FASST radios will work through the Interlink controller or not.

Overall I think you're on the right track with regard to maximizing the value of your investment in a good trainer. Good luck and good shopping!