I'd forgotten about the old Nichimo Graf Spee. (Wow - my computer suddenly acquired the ability to do italics again in these posts!) That company's kits did indeed run the gamut from mediocre to outstanding. I don't recall seeing that one in particular, but I'm sure honneamise is right about it.
The Italeri Graf Spee that I built was in a Testor's box. It did indeed come with a Heinkel biplane; the instruction sheet included instructions to convert it (well, sort of) to an Arado. (I think I was supposed to glue a piece of scrap to the top of the fuselage to represent the canopy - and add an additional blade to the propellor.) I did mine in 1938 configuration, with red and white stripes on the turrets, the big Nazi eagle on the stern, the coats of arms on the bow, and the Heinkel. (I've always found it ironic that two of the most awful, repressive governments of the twentieth century, Nazi Germany and the Soviet Union, built some of the world's best-looking warships. The Graf Spee, in her spit-and-polish peacetime guise, certainly fell into that category.)
The current Revell Germany website specifically states that its 1/720 Graf Spee has an "Arado seaplane." I'll believe that when I see it. I suspect the kit is nothing more or less than a reissue of the Italeri one.
There are, of course, several possible sources for Arados. A 1/700 one would work fine on the Italeri/Testors/Revell Graf Spee. Any 1/700 Bismarck, Tirpitz, Scharnhorst Gneisenau, or Prinz Eugen probably could be raided. The recent Dragon Bismarck and Tirpitz even have Arados molded in clear plastic.
The Gold Medal Models WWII German set of photo-etched detail parts includes lots that will work on the 1/720 Graf Spee: railings, stack cap, ladders, hose reels, etc. And a set of float struts for the aircraft. Installing them is pretty fiddly, but the difference in appearance is worth it. The only thing I wish had been added to that set is the plaque with the word "CORONEL" on the front of the bridge structure. (The Battle of Coronel was Admiral von Spee's great victory.) It shows up prominently in photos of the ship - and would be an eminently practical subject for photo-etching.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.