I've done the same but in a different order, I have the 5Diii and just bought a second hand 1D IV(wanted the f/p/s, but also the 1.3 crop) for Au$3,300.00 (the prices you mention seem excessive) with low shutter count, only problem was the sensor was filthy, now I'm saving for a 500 f/4L.
Well I made my final sale decision today. I did not go with a second 1DX, but rather kept my 1D Mark IV and sold my 1Ds Mark III and a 5D Mark III I had earlier. I really like the colors and extra reach with IQ on the 1D Mark IV for outdoor sports. So yes, one 1DX was probably worth the 6800 but I decided the 1D4 was so good I wanted to keep that, and then the 5D3 for weddings/baby photos and quiet sports like tennis and golf. Either way on the 1D4 you won't be disappointed, should you choose that route.
I decided to go with a 500 f4 instead of the dx and I'm now looking for a 1d3 or 1d4 ! The 4 would be nicer with the 16 mp but most with pretty high shutter count seem to all be $4,500+ when new they were $5,000 I think lol but thanks for your post!
What's the AU to US conversion lol? But I'm aussiming that's still a good price! How do you like it?
Au$3300 =US$3455.76. approx. all the used 1D IVs on Ebay at the moment seem to be going for about that- give or take a few hundred.
Loving my 1D IV so far, although I haven't had opportunity to use it a great deal yet. Shots I've missed have been due to user error, it loves the best lenses and performs well with those, doesn't much like my Kenko pro 2xTC