Not a great deal really. Headlights can be made to point tje right way, you'll need a rear fog light if there isn't one and a speedo that reads in MPH (although you might well get away without). I presume you want to register it here? If so, make sure you have all the paperwork from Spain, get it MOT'd here then off to your local DVLA office.
Relatively straightforward. Hopefully.