Normally i would say not. I have not used Mental Ray, but most other 3D rendering packages use cpu power and many use the video card as well(I have found CPU rendering is a bit more consistent across machines).
Nukemaster is probably right. Start by making sure you have plenty of memory. Once you get the machine, check your disk activity light while you're rendering something. If the light isn't on just about solidly (indicating LOTS of disk activity), then an SSD won't help.
If the light IS on solidly, it still doesn't necessarily mean that an SSD is the best solution. You may be paging heavily because of insufficient memory, for example.