Dear Dave,
You can purchase without using a lawyer, but it is not usually advisable to do so. If you are buying from a close friend, whom one assumes you can trust, it is less of a problem. However buying a property in Spain can be complicated so at the very least use a gestor to help you organise the purchase. You need to gather and check all the relevant paperwork before the transaction, make sure everyone has power of attorney if they can’t attend in person, etc. etc.
You can make payment anywhere in any currency. However this will have to be demonstrated to the notary. Also your friends will have to pay the corresponding taxes in Spain in Euros, as will you. You will also have to pay a 5% withholding tax to the Spanish tax authorities, so you will need some funds in Spain for the purchase.
You can register in your son’s name, but you may need professional help (lawyer or gestor) sorting this out.
If you want a little bit of legal help, charged by the hour at a reasonable rate, rather than the typical fee of 1% of the value of the property, then let me know and I will put you in touch with an appropriate lawyer.
Regards
Mark