If you ask me i would say archicad 19 is performance update and you can live with AC18 without any problems...If you have modeling problems in AC18 you will have them in AC19 too, so don't worry, workflow is the same !
Other architects who works in AC19 will not be able to do anything you can't do in AC18.
Get yourself AC18 and SSD...especially because AC18 is more polished then AC19 (bugs)