Welcome Renee!
And sorry to hear your old Lumix failed – but I think you’ll get a lot of enjoyment and great photos out of the new gear you are contemplating. I was astonished what I could achieve when I first starting using a DSLR after having an old Lumix probably similar to yours. And the DSLR I started with was very basic: a Canon 450D with a kit 55-250mm zoom. But that’s history…
I don’t have the 300 f4 so can’t comment on that but I do have the 80D and an EF-S 18-135mm STM lens to go with it. I also have the old series (mark 1) EF 100-400 L lens for bird photography. Here are some of my observations and experiences in case it helps you decide what to buy.
First the 80D. I’m very happy with it but I don’t have much to compare it with – just my previous 700D which I’ve now disposed of. The 80D has the latest generation Canon sensor with very good noise performance (= workable images at high ISO up to 1600 or above). This also flows through to a more sensitive autofocus sensor which is usable at lower light levels. It also has a customisable AI servo autofocus option with similar controls for Tracking, Sensitivity etc that you'll find in the high-end full frame DSLRs (eg the 5D and 1D). This is what you’ll need if you want to do flight photography. The AI servo focus speed and accuracy is reasonable in my experience but probably not in the same league of performance as the big full frame cameras mentioned. If you search for my recent images on this site, you’ll see some examples of what the 80D can do for both flight and still photography.
If you were unsure about the 80D, the other option would be the 7D mark 2. It seems to be a fine camera and used by quite a few members. The image sensor in the 80D is marginally better but the 7D mark 2 may be better in other areas of performance. For example, it has more AF points. Being a pro-level camera it is also likely to be more robust. However, it is more expensive.
Regarding your wildlife lens choice, I’d personally find a 300mm too short much of the time. The 300 f4 also isn’t a top performer optically as reported by independent test site dxomark (dxomark.com: Ian Wilson has written an article for the last newsletter on how to interpret the measurements on this site). The new EF 100-400mm mark 2 actually performs significantly better in terms of sharpness and probably has a better image stabiliser in it as well. It does cost significantly more though! And as you’ve noted, weighs more but not a huge amount. It’s about 0.5 kg heavier than the 300 f4. If you think you could cope with the extra weight (and cost), I think the 100-400 would be a much better choice. Do you know what the full frame equivalent zoom range was on you old Lumix? That might help your decision.
Finally, your choice of a 24-105mm L lens for your new camera body … as mentioned, I have the EF-S 18-135mm and mostly use it at the wide angle end. At 18mm it is equivalent to about a 28mm on a full frame camera and that is workable for me. If it wasn’t that wide, I think I’d feel limited. I’ve also been very impressed with the optical performance of the 18-135 as well as the image stabiliser in it. The STM focus drive is also virtually silent – important if you ever contemplate doing video. One downside with the lens is that it is an EF-S series so will only fit on the cropped sensor cameras – it won’t go on a full frame unlike the 24-105mm you are contemplating. I’d don’t know if that’s an issue for you though.
Apologies if I’ve made this too long-winded but if you have further questions, need help using the dxomark website (not the easiest to use), etc etc please post again!
Cheers
Glenn