You should be able to fit your OS (of course), applications, and "basic" home folder(s) on a 256gb SSD. With room to spare.
HOWEVER...
A lot of other stuff will have to be kept on an external drive.
This might include certain "large libraries" that would normally be kept in the home folder.
Such as:
- Photos library
- iTunes music folder
- Movies folder
This is not hard to arrange, the apps that use these folders will have no problems accessing them from the external drive.
Also... you will then need TWO backups:
1. backup of internal SSD
2. backup of external "primary external storage" drive
This could fit on a single large backup drive that's partitioned into two "pieces".
What I would do:
First, you need to "inventory what you have" to see what's using space on the OLD iMac.
Then, you need to start "moving stuff around" -- setting up the external drive as it will be.
What I would do:
I'd use either CarbonCopyCloner or SuperDuper to create a cloned backup of my 2tb internal drive on the external drive
Then -- when "new iMac day" arrives -- I'd use the setup assistant to "selectively migrate" things to the new drive.
Things I would bring over:
Applications
Settings
Home folder.... BUT... I'd go inside (using setup assistant) and UNCHECK the "documents" "movies" "music" "pictures" folders for now. That way, they'll stay on the external drive for the moment.
This would "bring over" basic home folder/account info, but leave all the large items "out" for the moment, until I figured out what to do.
A lot to digest, eh????