Film SLR.. are you familiar with the concepts of exposure? If so, it shouldn't be that hard. The 1D has Program AE (P) mode, so even if you aren't that confident, you can get started. I recommend P to most people who are starting out because it controls exposure just like auto, but still allows you full access to the camera's advanced settings (this is particularly true on Canon's other DSLRs). P will set your shutter speed and aperture to get what the camera feels is a proper exposure.
I wouldn't say the 1D is 'too hard' to learn. You might have to put in a little bit more work though, but it will pay off in the end. Understanding the tools go a long way to contributing to your chances of getting the desired results. Myself, I put my 1D in aperture priority, tweak ISO, and carry on
not too much to do..
Basically, it's about learning the control set of the camera, which you will on any other camera as well. The manual will help a lot, so make sure they give you one. If I'm not wrong, this is a 1D classic you are talking about? Since you said decent condition I'm going to assume it's an older, used 1D classic.
Your results in JPEG straight out of the camera are going to be fairly bland unlike say, a 30D. This camera basically demands postprocessing, since it has been set to produce images intended for postprocessing. However, there is a simple method of getting good results out of it (that I use extensively, despite all the tools available to me). Shoot RAW files, these will need to be processed by Canon's DPP software supplied free with the camera. Install DPP from the CD, then go to Canon's website and update to the latest version. You get Picture Styles, which automatically set tone curves, sharpening, color, etc. for pleasing results... just click the style you want, apply to all images and convert to JPEG. Essentially, you'll get quality on par with the 30D.
Also bear in mind the focal length multiplier. On digital, you will get a field of view (FOV) smaller than what you're used to on film. For the 1D, multiply it out by 1.3X - a 50mm would work like a 65mm lens.
45 point AF and 8fps are awesome when you shoot action, however you will probably want a telephoto a bit longer than 135mm for airshows.