183 day rule...

If I'm reading this correctly, you intend to rent in Spain before buying somewhere and  move here long term from day one. If this is the case, you need to apply for Residencia which you have to do for any stay over 90 days. Scroll down to the rhs side of this page to Jim's guides and then look for the heading. You will usually (this is Spain) need to show an income of €750 a month coming in to a Spanish bank account plus €7,000 savings, both for 3 months. You will also need to pay the first year's private medical insurance up front unless you qualify for the S1 as a UK pensioner or in receipt of an exportable benefit and change to a Spanish driving licence, preferably before the end of this year after which things may get a lot more difficult. Once you have residency you are no longer entitled to use the NHS if you don't have SIP.
If you move in the first half of the year you will become tax resident in Spain for the whole year and this could include having to pay Capital Gains Tax on a UK property sale.
Have a good look at any of Jim's Guides which you think may be relevant and if there is anything you still need to know, ask away.

I think you will need a TIE to be able to do that after end 2020.

And (others correct me if I am wrong), to get a TIE you need to put down roots somewhere within the EU.

Thanks to everyone for your helpful advice...

Looks like we're likely to be aiming for Spanish residency then !

Our main concern is if this will still be attainable when Britain leaves on the 31st December as there's very little possibility of my wife & l leaving the UK until next year..

We're youngish retirees Pete & have a €1800 work pension, but a largish savings pot from which to draw from..

Not forgetting that as things currently stand post the transition period a UK licence will no longer be exchangeable for a Spanish one in which case ALL arrivals will have to take a Spanish driving test within 6 months.

Being a 'new/probationary' driver could likely to have a significant effect on insurance premiums.