What I-mac computer specs should I look for for stable serious Arch Vis work?

Currently running:
Processor: 3.5 GHz Quad-Core Intel Core i73.5 GHz Quad-Core Intel Core i7
Memory: 16 GB 1600 MHz DDR3
Graphics: NVIDIA GeForce GTX 775M 2 GB

I’m only doing Architectural Visualization not game building. I’m building Models with Vectorworks and exporting .fbx and doing lighting and materials in UE4. From UE4 I am currently only exporting 2d Highres screenshots for rendering material. I would like to do walkthrough videos and possibly VR, but I’m never going to do much VR. So weaker VR capability is not a dealbreaker. I need to stay with Mac Platform for other business reasons.

With larger Models I’m crashing often. Load times are very very long. I would like to not only be stable but very fast if possible. I am considering an I-Mac Pro. I wouldn’t know how to configure it though and what specs would be important. I am hoping that the base model would be enough anyway as I have little appetite for spending over $5,000

Is there another iMac that would still work awesome? Is the i-Mac Pro necessary? What are the most important specs to consider?