Getin ghigh search rankings is not often as helpful as you may expect.
I', sure that you'd rather have 100 people look at your site who are seriously interested in staying at your property than 5000 casual 'click through' browsers who arrived there because they looked for "Spain"+"property"+"holiday"+"rent" and so on in Google.
One of the best ways of promoting small property letting sites is by letting your leaflets, brochures, business cards and so on circulate (with your web address on them of course) and then let word of mouth do the rest based on how clean and well presented your property actually is.
If you have a good holiday you tell everyone about it but if you don't you tell everyone and his dog.
There isn't really enough space to go into a full discussion of search engines rankings, but don't look on them as some sort of guarantee of bookings because they aren't.
There are other far more effective ways of promoting a property let than banging your head off every other holiday rental in Spain that Google currently has listed.
A simple site should not cost the earth but I'd suggest that you make something of it early on rather than look at a minimalist page or pages with a view to adding significantly to it at a later date. You'd be better off making sure that people visiting the site can see exterior and interior shots (high quality photographs), a list of places of interest and things to do in the area where the property is and why, above all, people should be parting with their hard earned to go and stay there.
Nothing will put people off more than low quality images or a tacky page or two along the lines of a "come and stay with us because" site.
If you want some specifics or have particular questions then feel free to bounce ideas around.