too late for the reply but i hope i can still help.
you mentioned you want to do group shots , cars etc. i think you should go for 35mm or even less than that. dont worry too much about F stop because you will be using f5+ inorder to get frame to frame sharpness.
then you mentioned about 85mm. on D7100. i have used 85mm on a FF and i can tell you that its very tight. therefore i would not recommend you 85mm if you feel 50mm is tight. you can do a simple test, just put your kit lens to 85mm and see if it suits you.
or otherwise you can just step abit closer with your 50mm