I would not begin to budget for it until you know if the problem exists. So you protect yourself by having an expert come over and check for moisture. Some physical inspectors have infrared meters and they will check areas of the home that will detect moisture. Others have moisture probes that they can stick in the wall to do the same. I like the infrareds a bit better but your inspector has to be good at reading them.
So you can insulate yourself to at least know what you are getting.
Now if you do find a place YOU like, you can get a contractor to come and give you a bid for doing things. If it is coring out the patio to put drainage in that that can be done. However if the patio is high compared to the home then yeah you may have to cut it away from the home and put a trench for drainage, then put grates there so the water will not puddle against the stucco. Not hard to do and the price will vary with the amount of work.
