Debutant

Hi everyone I've been following the articles and your experiences for several days because I'd like to upgrade my 3 old macs. An iMac 21.5 mid 2010 20GB of ram and 256GB ssd, a macbook pro 15 with dgpu disabled at the end of 2011 16GB of ram and ssd 512GB and finally a macbook pro 13 mid 2012 16GB of ram and ssd 512GB. First question: which systems are best for each machine Second question: if something goes wrong during the procedure, is it possible to restart my Mac on an external hard drive or I will have made a Bootable backup of my old system with CCC. Thank you in advance for your answers. Have a nice day