1996 Jeep Laredo oil pressure guage n' other things

My oil pressure guage will rise and fall with acceleration, and the idle speed falls well below the 900 rpm marker almost to a stall, as I depress the brake the rpm guage goes down below resting idle marker and when I come to a complete stop it jumps back up to just below normal. There is a hint of a smell that I believe is comming from my engine but i'm not sure? What could be the cause of all this? How much will it be to repair? Estimated? My check engine lite just came on is this all related? I'm not sure but I believe it to be so?