Hi Benjamin, welcome to the forum,
FBL technology is moving rapidly. The 3GX was discontinued some time ago and replaced by the Gpro, but that too is now discontinued. On all of Align's new 'super combos' the Beastx Microbeast is included (which is a nice controller but pricey if you buy it separately).
The 3GX is best avoided because there are much better options available today that not only perform better but are a lot easier to set up. It's been discontinued for quite a while now so any kits with the 3GX must have been on the shelf for a long time. The Gpro is a better unit but unless the deal is very good I'd give that a swerve too, I think they worked well enough but they aren't supported any more and there are not many users around to give advice. The fact that Align dropped it so quickly probably says something.
As luck would have it the AR7200BX is also discontinued, you are unlikely to find one for sale anywhere these days except in the second hand market. The replacement is the AR7210BX which is very similar but has optional (pay extra) 'stability' and 'rescue' (Horizon call this 'SAFE') features. The AR7210Bx is an excellent unit but expensive once you include the optional SAFE features.
Other options worth looking closely at are the Spirit or Spirit Pro these are excellent controllers that have all the latest stability and rescue features built in without paying extra. The MSH Brain is another good option but that's about to be replaced by the Brain 2 so you might be better waiting and getting the latest version if you went for the Brian.
If you are on a budget the 'Kbar' is worth a look at. it's a clone 'rip-off' of the Mikado Vbar but they work fine and are dirt cheap. They don't have the fancy stability or rescue options of the latest controllers but for $25 you cant complain!
Just ask if you need more info....
EDIT.. PS I moved the thread because you had put it in the forum for helis with 400-500mm blades (i.e. 500 size helis)... A standard '450' has 325mm blades so goes in the 300-400mm blade forum (confusing I know, blame Align)