from my experience most foundations are not 100% level. the general rule of thumb is 2-3 inches across the entire slab. I have never had an engineer use a water level. the guys I use always use the zip line. just remember that any slight issue that you encounter now you will more than likely be forced to deal with when you sell. of course in austin we do have alot of clay which does cause alot of foundation problems. it is a common issue in certain parts of the city. If it were me and I really liked the house and I planned on being there for a while I would go for it. Of course it couldnt hurt to get another opinion. Most engineers will do a free verbal. I wish you luck.