Offers are not legally binding and you should make some conditions such as:-

1. Check the Escritura (title deed) is correct and registered and the people who are selling the property are named on the Escritura.

2. Make sure the verbal description on the Escritura matches what you have seen (difficult in practice). You may need the assistance of a land surveyor (topografero) to provide a topograghical report on the site to denote exact boundaries and size of plot.

3. Check the catastral reference on the escritura with the catastral plan at the town hall. This is a complete plan of all the registered properties in the area and clearly shows your plot with boundaries and position of any buildings. This should tally with what you have seen and what is described in the escritura.

4. Check the Nota Simple. The Nota Simple shows the latest recorded details of any charges. There are many forms of charges – in Spain debts, charges or court orders are registered against the property rather than the person: these include hipotecas (mortgages), censos (ground rent or leasehold payments), usufructos (interests in the property), limitaciones
(restrictions on use),  multas  (fines),  cláusulas resolutorias (determinations, i.e.
decisions about the future of the property), or embargos (court orders for distraint or seizure).   

5. Have a survey carried out on the condition of the property (not legally required but advisable).

6. Check your finances. Budget for 10% on top of the purchase price for expenses (tax etc.). Make sure you can get a mortgage for what you want (amount, term, interest rate, monthly payments). Some properties are not readily mortgageable here in Spain such as illegal builds, unregistered titles, newly registered title etc.
We have good banks and brokers to refer you to, who will help you, are able to speak English and more than likely will save you money in rates and fees. For example Spanish banks charge less than the English banks who set up Spanish branches aimed at English buyers.

7. Check with the town hall for the urban plan (PGOU)  to check the zoning in your chosen neighbourhood and to see if there any planning applications are being considered. One of my neighbours paid a lot of money for his house because of the great views only to discover  one week after completion that developers are building a 6 storey hotel IN FRONT of his house. With this situation the house may only be worth 50% of what he paid for it ! 

8. Check what element of the transaction the seller wants undeclared (known as black money). Much less common than it used to be but nevertheless common. This is illegal but the Spanish don't generally like to pay tax if they can avoid it. This will effect the amount of mortgage you will be able to get and how much tax you will have to pay. This is sometimes done as a separate contract for fixtures and fittings.

It is not law to use a solicitor or conveyancer but it is a good idea. Solicitors will charge 1% and conveyancers about 0.3%. In the absence of a solicitor or conveyancer the agent will draw up a pre-sale contract (Pre contrato de compraventa) which will contain an arras agreement which means that should you back out thereafter you lose your 10% and if the seller backs out he must pay you your deposit back and another 10% penalty.
Monies should be paid into an escrow account so the funds are protected until the sale is completed. These accounts are unfortunately rare and if there are any doubts put a lien on the property at the registry to stop the seller selling the property elsewhere in between exchange and completion.
If you want to make enquiries before signing you can nominate a power of attorney (poder) to act on your behalf here in Spain.

At this point it is a good idea to apply for your Non-Residence Fiscal Identification Number (NIE- numero de identificacion de extranjero). You do this by presenting yourself to the nearest Police Comisaria with a foreigners department, with your passport, several photocopies and several passport sized photographs. Fill in the form and wait several weeks for your number to be assigned. You will need this to complete your house purchase. For the Costa Tropical this is in Motril.

Again this information barely scratches the surface and you will need the assistance of a solicitor or specialist gestior. This subject is also well covered in my favourite reference book, "You and the Law in Spain" by David Searl.