I don't know about the mini 3D in particular, but I don't have a problem with someone going to a fun-fly model for their second plane. Something like an Ultra Stick would be a more natural progression, but I say go for it.

If you plan on going with a fun-fly please dial down the control throws to tame the model a bit for the first dozen flights or so, just to get familiar with it. Also be prepared to stay ahead of the model while you're flying it. Fun-flys can get away from you pretty quick if you aren't prepared for it. Lastly, you should probably get an experienced pilot to go over it and test fly it for you. If you're confident that you can test fly it yourself go ahead and do so, but it's probably a safer bet to get a veteran pilot to test it for you.

About the radio: It's probably a good idea to put it in the Nexstar for a flight or two while you're putting your second plane together just to make sure it works, but it's not a huge deal if you don't. You won't be thinking about your radio while you're flying, and the sticks still wiggle just like your old radio.