For a variety of reasons and from practical experience please be very carefull when selecting a keyholder and look for somebody that is experienced and operating legally in Spain
Remember that the condition of the property, when people arrive is key to ongoing rentals and it is not an easy service to get right
Now I am not wishing this on anybody but If you have a break in and the Police attend with your keyholder and they discover that they are not providng a legal service and paying tax and National Insurance they can and do disallow your claim, this has happened
Typically a good service should cost you around €300 per annum for keyholder and regular checks, another €50 - €80 for a lean, including linen change plus some charge for meet and greet