Hi Albert,
My partner and I moved over on 30 November last, having sold our property in the UK. Since then we have set up the legal aspect of a business, got our Tax number, got on the Padron, got our Residencia and Spanish driving licence.
We used Ana and Alejandro at Salvador Tax & Legal here in Mojacar Playa. Alejandro is based in their Almeria city office. Both have been excellent, and have got everything done for us very quickly and efficiently despite the Christmas/New Years break.
In order to apply for Residencia, you will need to already be on the Padron. For Padron, you will either need a rental contract of no less than 6 months remaining at the time of application/ or written authorisation from the owner (the town hall have a form) or proof of ownership of a property, and it will take approx. a week to come through. Residencia can be started by Salvador but you will need to go to the Extranjeria office in Almeria in person. Salvadors have a very efficient lady from their Almeria office accompany you. She is fabulous and had us in and out in 35 mins flat, Residencia cards in hand.
You will as others have mentioned have to at least apply for Residencia before 29 March. There are 4 key criteria - Padron, NIE, Proof of income and proof of medical cover. The new business set up (Autonomo) was sufficient to prove income/medical cover. Otherwise, you will have to buy private medical cover (or get a S1 from UK NHS if you are pensionable age) and provide proof of funds (though UK bank statements are now sufficient - you don't need to move a large sum into Spanish bank account if you prefer not to).
You can apply for your NIE in the UK either at the Spanish Consulate in London or Glasgow, person. We did this before we came and it turns out this was the easiest and fastest route, approx. 10 days.
The remaining time is tight, so a good Gestoria like Salvadors is the most assured route, though I would not leave it any longer to be honest.
Best of Luck, it might seem daunting, but Salvadors will navigate you through it.
Jennifer