Replying to What nikon lens to buy

Hi guys, after deciding to stick with my d5100 instead of buying a canon t5i (dont have enough money right now) I decided I will invest some money into a new lens. Im looking for a fixed aperture lens for filming mostly follow cams and other cinematography. i have the 18-55mm kit lens as well as the 55-300mm lens. My price range would be ideally under 300 or somewhere around that. let me know what you guys think.

also would i be able to buy a canon lens and stick it onto my nikon. cause i might want to make the switch to the t5i one day...