If it doesn't exist any covenant how is the distribution of the fees when I am buying a property ?
Spanish law dispose the vendor has to pay the Registry Land fees for cancel charges, Notary fees and Plusvalia Tax (Municipal Urban Land Gains Tax). Buyer will pay the first copy of the title deed, Transfer Tax, VAT in new properties or ITP AJD in resales.

Where and when can I collect the keys to my new property ?
The keys will be available after you sign the deeds. Do not therefore expect to be able to obtain the keys before but you can if you wish agree with the Seller the collection of keys before or more usually they can be collected from the Agents through whom the sale has been conducted.