Property sells my unit too property investments, and gives me no notice of the sale. I only receive letters on my door from "investments" Doesn't original landlord have to give me notice of the change even though rental agreement is to stay the same?? Doesn't a new agreement need to be signed?

No, there is no "new" rental agreement, the agreement is between buyer and seller and the buyer assumes the contract and becomes the landlord.
So the old lease or rental agreement is still in place. They are bound by the terms of the new least. All they have to do, is notify you if there is a change in where and who you pay the rental money to.