I shoot a 1D4 & 7D2 at R/C events and both are perfect for my use but all things considered the 7D2 may be a better idea for your needs. Used or new it will do the job. At most of my events I need prop blur which is easier to get on models than full scale but they fly much closer than full scale & almost as fast as full scale does when making show passes for us photographers. This is from the 7D2 with the Sigma 150-600C @ 205 mm at 1/500 sec and the on board GPS based telemetry said the pass was at 191 KPH (118 MPH). Full frame first, then a tight crop of text that's about 1 inch high second.
A friedn of mine with the lowest beginner cropper camera and my Tamron 150-600mm, first time doing and air show! 
@chief54 wrote:would a 1d MIV with moderate wear for $1200 be worth it. BH Photo has one for that price, I have also seen other 1d MIV on other sites in the same ballpark on price. I can not afford a 1dx so I thought this might be an option, I shoot birds in flight and right now I am using an 80d with a Tamzooka 150mm-600mm and have got soome good shots. Is this worth the faster shutter speed but lower megapixels, among other things Any thoughts
As good as the 1D4 is, I would stick with the 80D. I think investing in a Canon battery grip would be the wiser choice. Do not get me wrong, the 1D4 is a great camera, which can capture some amazingly clean photos.
BUT... The 80D has a better AF system, with 27 f/8 AF points. I do not think one is any better than the other when it comes to noise. But, the 80D has 50% higher resolution.
The slightly higher frame rate of a 1D4 is not as significant as some might want to believe. I set “Image Priority” in my cameras for “Focus Priority”, which means the focusing speed of the lens may become the limiting factor when it comes to frame rate.
If you decide on a 1D4, you will not be disappointed, though. I have a higher keeper rate with my 1D4 than the 7D2.
