I am assuming that you bought this for resale because if you were going to keep it, you would only have problems down the road and it would cost you more to fix them.
I am going to be kinder than plumber Tom but, still agree that you should do the job right from the start. tomh is also correct about getting a dumpster and going at it. You won't know the extent of the damage until you tear it out and look at it.
Now is the time to upgrade all systems. You will get more return for your dollar as well!