If the tenants are responsible on other matters then having a cat shouldn't result in any change to how they treat your property. They should have a scratching post etc. because they can damage the sofa otherwise but that would be their responsibility.
You might want to mention the cat to them, since you noticed it and it was in the lease and to keep everything out in the open.
As for being dirty, a cat will use a little tray and keep themselves very clean (can't say the same for dogs but at least they're friendly IMO!), there may be hair but no lasting scent or markings, bar the scratching...