Underhood plastic "stuff"

I'm talking about on the driver's side, from behind the headlight to over the battery. I hit a deer with my 5 last year, and lots of the plastic stuff is broken, and rattling like mad. Can I just remove it all (and the engine cover) or is it necessary? How about sourcing new/good used ones? I would imagine with all the heat and vibration, anything used will be just as broken as what I have. THis 5 is the newest car I've owned, so I'm unfamiliar with all the plastic covers, etc under there. I know I need another bumper cover, and whatever the panel above the radiator, but I'd like to just remove everything else under there unless I'm gonna cause trouble doing so.

All plastics under the hood serve some purpose. The tubing that goes to the battery provides air in the event of excess gassing should the battery get overcharged, sealed battery or not. Debatable if you “need” it since there is still an opening to the battery and the engine bay gets enough turbulence that air can still get in. I've been without it for almost a year but do plant to put it back on. The other plastic pieces are mostly “top” covers to prevent water entry to electronic parts. The engine cover is probably the only cosmetic piece, I think.

Horse hockey, it can all go with the possible exception of the battery tube thinger. The part you are most likely to miss is the belly pan, and even that won't cause the car to burst into flames if you remove it. That being said with most of the underhood plastic removed you will find yourself spending more time cleaning up underhood, as road crud gets on everything without the gaurds in place.

The panel above the radiator part, if you mean the radiator support then I would say that might be the most important part to replace or fix. Include a pic of the damage and people should be able to tell you what really needs to be replaced