Hi Tony
I have a BX1850, basically one version back from yours. It's rated for either the 48" or the 54" mower; the guys at my dealership advised against the 60" unless you have the 22-hp or up engine. I got the 48" because it's only a bit wider than the tractor and so the better half would be less likely to snag it on something when mowing our walking trails (notice I said LESS likely . . .
). The 18-hp feels good with my mower deck, should be ok for you even at 54".
I have not used a "Cyclone" picker-upper; we just pull our axle-planetary-drive bagger behind the JD X300 for leaves, not for grass, so no help to you there.
BTW, if you buy the mower deck "in the crate" instead of fully assembled, you might get a deal on it - my dealer was good that way, and it made it easier to get home using the crate to unload it. Same thing with my tiller.
Hope this helps.