Potential water damage issues with siding being in contact with patio
Codes vary, but generally speaking wood and related exterior products must be 6" or so above the ground. I'm not sure whether a concrete slab is treated the same as the ground in this case. It's probably moot in your case as you're now beyond the point where you'll ask the seller to deal with it. 

There isn't a whole lot you can do assuming that you have typical wall sheathing behind the siding. Replacing the siding with something more durable (white or painted vinyl planking, for example) will prevent visible decay, but doesn't help with rot behind. 

To offer further suggestions I'd need to know what the wall looks like behind the siding.