OP:
The easy answer here is:
Don't buy an iMac with a 512gb drive.
Get one with a 1tb internal drive.
Doing so will make things easier.
REASON WHY:
With a 512gb drive, you're going to have to "cut down" the amount of data that gets transferred to considerably BELOW 512gb.
That's because a Mac needs sufficient "headroom" in which to operate -- I reckon at least 20gb or so.
Unless you have A LOT of stuff on the old iMac that you can "off-load" to external drives, or just delete...