ArchiCAD does not use the GPU for rendering, only for 2d display and OpenGL in the 3D window.
If you want to use GPU rendering you will need to purchase Octane Render plugin but it is not usable on your iMac or really any new Mac as it requires Nvidia cards for Cuda which is not available on the Radeon cards used in current Macs
Scott
The iMac is OK as for intermittent use of ArchiCAD and 3D rendering as long as you have enough RAM.
If you were using ArchiCAD every day I would suggest more of a workstation class machine but Apple do not currently make a workstation worth buying, the current MacPro is very out of date and very expensive (A$10.5k for MacPro equivalent of A$4k HP z440 workstation).
Scott
For occasional small project use it is fine. The RAM is an easy upgrade and I recommend it. Not so sure it is worth cracking a new machine to put an ssd in unless you are experienced with that kind of thing.
Rendering is slow no matter what you have so you need to make time for it either way. The question is whether going from a 30 minute render to a twenty minute render is worth the money.