I have recently returned to photography, having bought a Canon EOS 60Da with 55-250mm zoom, a wide angle lens, and the standard 50mm. I've never owned a digital camera before; the last (best) SLRs I owned were a Canon AE1 and AV1, and I was wondering how you rate the Canon 60Da. It's not a camera I would have particularly gone out and bought, but a friend's uncle passed away and I was offered the camera and accessories at a very reasonable £250.
I shot with an AE-1 for several years, and now use a 60D exclusively.
The main difference between a "normal" 60D and your 60Da is that the internal infrared filter on your camera has been removed so the camera may be used for astrophotography (hence the "a"). Other than that, the 60D and 60Da are the same. For more info on the Da model, see Do I need a filter to use my Canon EOS 60Da for daylight photography?
My understanding is that the auto white balance will not be correct with the 60Da, so you will see color shifts. If you are not going to use the camera for deep-sky astrophotography or infrared photography, you might do better to sell it and purchase a used 60D instead (probably profitable) since this is your first DSLR. However, if you shoot in RAW format, you can correct most white balance issues in post-production - your computer is now your darkroom.
Since you already know the basics of photography, I believe you will find this machine to be a heck of a lot of fun. With digital photography you don't have to worry about developing film, so shoot as much as your SD card can carry, and you aren't stuck with one film speed. The setting for sensitivity is ISO, which is the same scale as film - it starts at 100 and can get as high as 12,800 on this camera, but I wouldn't recommend going past 3200.
This camera has a "crop" sensor, meaning the sensor is smaller than a single 35mm frame, so the lenses you have will be different from the AE-1 or AV-1. Multiply by 1.6x to get the 35mm equivalent. For example, a 50mm lens will have the same coverage as an 80mm lens would have on an AE-1.
I suggest you ignore the "idiot-proof" settings, the flower etc, and start by setting the camera to Av. That is aperture priority, similar to your AV-1. You can use the dial nearest your right forefinger to change the aperture and the camera will control the shutter speed, and the ISO if it is set to auto. You can use the dial on the back with your thumb to set exposure compensation up to 3 stops in either direction.
The Tv setting is shutter priority, with the same logic, and of course M is manual - you control both shutter and aperture manually using the finger dial and the thumb dial on the back.
One tip - if the 50mm is the f/1.4 version, you will find it sharpest and with least chromatic aberration stopped down to f/2 or smaller, but it is a dreamy portrait lens at 1.4.
Definitely read the manual cover to cover, and have fun! You got a fantastic deal - essentially getting all that glass for free - and I predict it will renew your interest in photography completely.