@NewInLinux -- please use English as said before. That will ensure more helpful comments and more users inclined to help. I am from Spain and I understand what you posted, but it's not right to leave messages in Spanish when you see that all the forum messages (every one of them) are in English.
I had a Macbook Pro Retina 13", mid 2014, which I think it would be more or less the same era as your iMac.
I used with success Elementary and Ubuntu (use x86-64 variants) on that Mac, and either of them will work for you. However, I had the following issues, that I believe you will have, and you will have to troubleshoot them:
- Have a wired network connection handy. Do not rely on the wifi to work at first attempt --it didn't work for me back then. You will have to research a bit what driver you need, and then locate what package the driver is provided by, and then install it for the WiFi to work.
- The Facetime? camera didn't work by default. I remember I had to compile a driver from source, but don't have any notes from that one.
- You will need the nvidia drivers for your card.
The above means that you will need a DVD ISO image with all non-free drivers included, but that doesn't mean everything will be already there for you except maybe the NVidia drivers.
Regarding the wireless devices, I have never had issues with bluetooth mice, but also, never tried the apple wireless devices. I bet they will work, but don't know.
Regarding the secure downloads for the software, yes, all (decent) distributions secure their package repositories not only by HTTPS connections, but also digitally signing all the packages. On the other hand, if you download a package from other sources, you must verify the signature and the checksums yourself.