You're welcome. If you wanted to keep your current 100-400mm lens, and have it capable of autofocusing, then you'd have to go with one of three options: 1. Get a Canon EOS-M mirrorless model - it has the same APS-C sized sensor as entry-level DSLRs, and via a simple adapter can take all Canon EOS lenses. The camera & adapters would run you in the $500 or under price range. 2. Get a sony E-mount mode, also APS-C sensors, and a Metabones or Kipon autofocus adapter. They will work with autofocus and stability on Canon lenses, but are not very fast at autofocus, so shooting birds in flight would be out of the question. You can get very good models of these cameras like the A6000 for under $500 body-only, or $600 with a kit lens, and add the adapter for $250-350. Or 3. You can go with a Micro 4:3 camera body, and similarly use one of the Kipon/Metabones adapters for Canon lenses. Note that with the Micro 4:3 cameras (Olympus and Panasonic) you can get plenty of models for well within your budget - but some higher end models will exceed your budget. With the Sony or M4:3 cameras, you could get a kit lens and a moderate zoom (ie: 50-200mm) for well within your budget, and still have room to buy adapters for either telescope or scope use, or for Canon lenses.
Mirrorless cameras in general are known for their ability to adopt for use with any lens made - they have super-short registration distances between sensor and lens, shorter than any previous lens mounts made for rangefinder or SLR cameras - so all you need to adapt another manufacturer's lens to these cameras is a 'spacer' ring that places the lens the proper distance to the sensor to focus. Many mirrorless shooters adapt old film SLR and rangefinder lenses from the 1950s through 1990s - all manual focus, manual aperture rings. Newer lens mounts that have autofocus, like current DSLRs, can be adapter too, and some more expensive adapters have been made that incorporate the ability to autofocus and use the lens' stabilization too. As for how well they can 'double up' with DSLR lenses - that tends to be better when you stick to the particular manufacturer. So: When you buy the Nikon Series One mirrorless camera, they have their own adapter that mates it to Nikon DSLR lenses with full functionality...any other lenses you adapt will be missing metering, or autofocus, or aperture control, or something. If you buy a Canon EOS-M mirrorless, their adapter you can buy gives you full functionality with Canon EOS lenses. If you buy a Sony E-mount mirrorless, their adapters you can buy give you full functionality and speed with Sony Alpha or Minolta A-mount DSLR lenses. So while mirrorless cameras can take ANY camera lens from any mount, they tend to always work best with the adapters made for their own manufacturer's lens mounts, to maintain fastest autofocus. Any of them are easy to attach to a scope, as most of these mirrorless mounts have scope mount adapters. There are typically two ways to go with scope adapters...one is if your scope has a removable eyepiece, and typically would be a T-mount for adapting to cameras - you can get a T-mount adapter for your particular mirrorless camera body, and attach the camera directly to the scope via the adapter. Or the other method is to get an adapter that allows you to use the kit lens or a simple prime lens attached to the camera, then mate the lens to the eyepiece via an adapter that attaches to the lens' filter threads.
Hope that helps explain a bit. I think this type of camera can be a good versatile choice if you're looking for something that can attach to bigger, longer lenses when you need it to, shoot with small or prime lenses only when you want to keep it compact and portable, or attach to a spotting scope or telescope for specialty use.