Hello mate,
As a Mechancial Design engineer who uses AutoCAD on a daily basis I could be of assistance here with a bit more info.
What are your planned uses for said equivalent? Does it have to be 100% free or are you willing to spend a small amount eg..>£50? Is it only 2D your after or 3D too?
Feel free to PM or what not, this subject is in my real comfort/expertise area now, unlike photography where I am still the noob!