I just saw this thread. Too bad, because maybe I could have helped you out a bit. There IS
a bagger for the Z700 series, but most dealers are either too lazy or too stupid to know about it. My dealer swore up and down that there wasn't. I went to Deere's site and found it and sent him the link. He said that they weren't even aware of it, and called Deere to confirm it. This is a very large dealer too, with lots of stores in all areas of sales from small to the largest ag equip.
I have a Z720 and it is an excellent machine. I mow professionally with it.
As for the extra 4 HP- you'll wish you had it once you run a bagger with yours. They take a lot out of a ZTR engine along with the hydros. I almost went with the 910/54", but ended up getting the 720/60".
Take a look at the Kawi website. The engine you have (FX651V) has been downgraded from 22 hp to 20.5 actual HP under the new, more accurate system that they are all going to, to rate engines. That's not a lot to run the deck, hydro's and bagger.