Apple computers for rendering and I.D. modeling are pretty expensive and they don´t deliver the performance of a pc of the same price. Value/Price ratio here.
I will go for a computer with the higher ghz speed of the CPU I could buy with an SSD as hard disk. Rhino and Vray use a lot of the hard disk in certain situations.
16 Gb of RAM for the most of the projects are ok but if you can buy 32 would be nice.
Laptops get outdated very fast in these days, so I will not spend more than 2k on it. Just my thoughts.