Questions about loading new iMac

Hi, I just bought a 2019 iMac 21.5" with an old fashioned hard drive. I'm in Argentina, where I can't get it configured to order, so I will buy my own SSD and have the drive replaced by an Apple-certified technician. My question is, should I transfer my data onto the computer now, or wait until the SSD is installed? Are there pluses and minuses I should know about? Thanks for your help!