This is is more of a hardware question, so I'm moving it to that forum.
You're probably aware, Jed, that Apple has not updated any of their computers in far too long - so if you have to buy now, you'd be buying old tech. Wait if you can, but if you cannot wait, save some money on refurbished via Apple online (for example).
See the age of the current offerings here:
At least the iMac isn't quite a year old yet. Retina MacBook Pro, Mini, Mac Pro are all ancient with updates hoped for / expected this fall.
Always get the best video in a non-upgradeable Mac. With memory, the 27" iMacs have user-installable memory, so you can save $$ by getting a base model and updating with 3rd party memory. The more the better.
I recommend Time Machine for your general backups, and cloning prior to any operating system upgrade (e.g., from El Capitan to Sierra) in order to be able to reboot into your old OS in case you run into problems. Cloning is not a great method for general day to day backups as you typically only have one backup of every file .. while with Time Machine if you realize that you messed up and need the project as it was from x days ago, it is trivial to restore it.
If you are using BIM Server, then follow the separate backup strategies recommended for it as well. There is a separate forum for BIM Managers that addresses those issues.