Changing the state of registration of an aircraft is, put simply, straightforward (sic). Purchasing a UK aircraft that has a history of previous EASA compliance shouldn't throw up any headaches. But ....... buyer beware. There will be a number of UK registered aircraft going back before EASA that have not been required to update: i.e. engine age which can become problematic when changing the state of registration. You will need to change the country of registration decals of course which can be expensive if this requires a paint job. The engineers you choose will charge you for a full required annual inspection plus any charges for rectivication. The Spanish authority will charge for registration and the issue of the Cof A and Radio Licence. Whatever the spanish require you will have no control.
On the other hand you have the whole of EASA land to shop in. Simply buy it and fly it home. All EASA certified engineers throughout the world can then maintain it and caryy out the annual certification paperwork required with the current aircraft country of registration which you will not have to change.