Having done several major floor projects myself, this one could be rough. You will first have to remove all of the tile, cement board (if they used it), and every speck of thinset from the underlayment. You can install hardwood on underlayment that is off-level but smooth; you can not install hardwood on underlayment that has bumps, low spots etc as your floor will become a squeaking nightmare in no time. There are two ways of approaching this if you are set on hardwood: rip up the tile and underlayment and then install new 3/4 CDX plywood underlayment (the better option), or rip up the tile and cement board and attempt to sand down the thinset with floor and belt sanders. That will be an absolute dangerous dust mess with no guarantee that you will get a usable surface. Having done 1000 sq ft of new underlayment and hardwood, it was a beast but the floor is stunning. It was also very expensive and that was with me and buddies doing the work. If all of this sounds like too much work, you are not out of luck as there are two options: laminate flooring (Pergo) can be installed over tile and it actually looks pretty good. Next, you can buy what's called engineered flooring, which is real wood on top of a plywood sandwich. Engineered wood can be floated, meaning no nails or glue, or you can probably glue it directly to the tiles. You should consult with the manufacturer before doing this, as once you glue that stuff down, it ain't coming up. One of the best engineered brands out there is Kahrs. the neat thing about engineered wood is that if it is properly installed, you can not tell the difference between that and hardwood strip flooring. One issue that you will not be able to get around is that both laminate and engineered wood, if installed over the tile, are going to add from 3/8 to 1/2 inch of height to your floor. good luck.