Aiken 96GT

Hey guys whats up. I just bought a 96 GT auto. been wanting one for many years now, finally able to afford it. However, this one has a few small hints of major repairs in the future. First one i figured out ok, it was leaking trans. fluid, but it turned out to just be the grommet for the dip stick tube.
When i first start it up, it smokes (white smoke). One guy told me rings are burnt out and/or valve seals are bad, allowing oil into the cylinders. Either way, has anyone had to fix this problem? it has 151K miles on it, and i was told the motor was rebuilt. I'm a little leary about that rebuild if its smoking, but its too late now, i already bought the car.
Next thing is a thumping i feel in the floorboard while sitting at a light. i havent dug below the surface to find the problem, but it goes away when i take it out of gear. monday i will be able to get the car up on a lift at work, so i'll be able to check it out good then. Just thought i would ask other mustang owners before-hand in case somebody knew what to look for.