1) if it's between the X52 and the X56, go with the X52. I heard the X56 struggles with build quality and has poor ergonomics. If you do decide you might consider something else, take a look at Thrustmaster. Especially the T16000M and the TWCS are excellent value. (And i'm usually a Logitech fanboy)
2) Considering you've already flown warbirds, you have a better understanding of flight dynamics than most beginners, so really you should take the plane you're most interested in. You might be able to get the fullest potential out of the F-5 quicker than the F-18, but that's just because the latter is way more complex and versatile. All said, the F-18 is an easy aircraft to fly, so I'm pretty sure you'd have no problem with it.