Bird watching camera, need some advice!

1: I don't know Canon cameras very well, but I'd 100% go mirrorless (from any brand) instead of DSLR. Modern mirrorless cameras have way better autofocus, faster frame rates, are smaller, and neater. Check out the Canon EOS R7, just announced this summer. Up to 30 frames per second shooting (with electronic shutter, 15fps with physical shutter), the entire sensor covered in AF points (more than 600 of them) and it has bird eye AF and tracking. It's about €1700 in shops in Ireland, but seems to be out of stock at the moment (I think it actually hasn't been released in Europe yet , so they're on pre-order). E-Infinity (grey market) have it for €1569 https://www.e-infin.com/eu/item/10895/canon_eos_r7_mirrorless_digital_camera_(body_only)

Here's a video review specifically from a birding persepctive that seems very positive:

Note that it's not a full frame sensor - it's APS-C - which is smaller. That can actually be an advantage in bird and nature photography, as it will give your lenses more effective reach (a 500mm lens will give you an effective view of an 800mm lens). However, it uses a different mount to the EOD 5D you mentioned, so you'd need an adapter to use the lenses your family member has (assuming they're EF lenses that they have). Also, Sigma only make lenses for the EF mount, not the R7's RF mount, so you'd need the adapter for that 150-600 lens too.

EDIT: actually, if you are thinking of getting it, check this out. Photospecialist in the Netherlands have the R7 for €1519, and they throw in a free EF-RF mount adapter. That's a really good deal. Photospecialist are a 100% legit EU seller, not grey market. They're part of a large chain of camera stores in Germany, Netherlands and Belgium. Again, it's not in stock there because it hasn't been released in Europe yet, but I'd keep an eye on this page if you are interested in that camera: https://www.photospecialist.com/canon-eos-r7-body

Speaking of lenses, no matter what you go with, I'd be aiming for a minimum of 400mm at the long end, and reaching for 600mm if you can. Birds are small and far away, so you can never have enough reach. The downside is that unless you're spending astronomical money, you'll be limited to f6.3, f6.7 or maybe even f7.1 at those kinds of lengths, so you need relatively good light (you'll run into trouble in the depths of a woodland on a dull day, for instance). A camera with good high ISO will help, as will shooting in Raw and using a de-noise plugin (like Topaz Denoise AI or DxO PureRAW 2) in post to clear up the image.

Personally, I use Sony mirrorless cameras (the A1 and the A7iii). I'd highly recommend the Sony A7iv, but it's about €2,800. Sony's autofocus is class-leading, and the lens availability from Sony, Sigma and Tamron is second to none. But you can't go wrong with Canon (or Nikon) either, just that I'm not as familiar with those models.

2 : If you wanted to go used and still have some peace of mind, you could try mpb.com - they give a 6 month warranty on everything the sell. Personally, I'm comfortable with buying grey market from either e-infinity or Panamoz. e-infinity are usually cheaper, but only give a 6 month warranty. Panamoz give a 3 year warranty and have excellent customer service. I've bought lenses from e-infinity with no hassle, but when I was getting my Sony A1 (spending 6 grand, but saving over a grand on what it would cost here), I went with Panamoz because of the warranty. I have no hesitation in recommending them even with massive purchases like that. Edit: but as I mentioned above, also check out Photospecialist. Most of the time they sell at around regular retail price, but sometimes they have crazy good deals. I got a brand new, absolutely perfect Sony 16-35 2.8 GM lens from them the other week for an absolutely amazing €1600. The usual price is €2400 (they've gone back up to that now). Even second hand it's going for €1800.

3: The SX70 HS doesn't look like a bad camera, but a good interchangeable lens camera with a decent lens will definitely give you a better experience and better, sharper photos.

4: I think most physical camera shops will let you trade in gear, but I've no idea what kind of deals they do. I've sold stuff in the past on eBay (hassle), to family members, but more recently I just sell things to mpb.com. It's an absolutely painless process. All you need is a box. They even arrange the courier to pick the gear up.

Best of luck with your decisions!

Currently the best birding lens is probably the OM (Olympus) 150-400mm f4.5 combined with an OM-1. A bargain at just €7,000.

For birding, you should be spending far more on the lens than the camera.

If you're looking for a budget entry to birding, the Canon RF 100-400 looks like it will be "satisfactory". On full-frame 400mm is too short for birding, but on the R7's APS-C sensor, you'll be getting a focal length equivalent of 640mm, which is much more usable. f8 at the long end is a bit of a concern - you could end up struggling in some lighting conditions. Also, with cheaper lenses you tend to need to stop down a bit to get the sharpest image, and you don't have very much leeway with f8. On the other hand, reports suggest that the AF on that lens is very good, it's easy to handle and if you go into it accepting it's compromises, it seems it will do an OK job.

The Sigma has a more respectable f6.3 at the long end, but lacks stabilisation (which can really help with hand-holding a long - both in terms of focal and physical length - lens) and I'm not sure if the focusing would be as fast as the Canon.

Unfortunately, it looks like the obvious recommendation - the Sigma 150-600MM DG OS HSM Contemporary - has a compatibility issue with the R7 relating to AF: https://www.youtube.com/watch?v=0lmjC07v8yI Which is a pity, because otherwise that's definitely the lens I'd recommend.

How about the Tamron 150-600 (€1349)? https://www.photospecialist.com/tamron-150-600mm-f-5-6-3-di-vc-usd-g2-canon-ef It seems to me to be the best "affordable" birding lens for Canon that works well with the R7. Not may reviews on YouTube, but this video spends a bit of time on it, goes though the pros and cons, and it comes out pretty good: https://www.youtube.com/watch?v=XnvuGbBKqNU.

I'm not a Canon shooter at all, so I don't know these lenses, so if I were you, I'd spend a bit of time on YouTube looking at the reviews that focus specifically on birding and nature, and for both lenses. And there are a fair few of them. But I will say from personal experience that a mediocre lens only leads to frustration when birding. It's a hard genre at the best of times: you need light, weather, the location and the birds all on your side, and that's before you've even unpacked your camera. Then it's demanding of your gear, so having your lens being the weak link is something you're best trying to avoid.

Cool! Hopefully it arrives early. Enjoy it when it does!