Hi Folly. Are you wanting to tow with an British registered car or a Spanish registered car?
If British. Find the weight of the caravan ( on the manufacturers plate, usually on the drawbar)
then check that your vehicle is legal to pull that weight.(if ness send me the weight and vehicle details, I'll check it for you)
I presume your vehicle would be British taxed mot'd and insured (Check ins covers for towing)
Check Caravan club site for hints and tips on touring in Spain.
You should then for all intents and purpose comply as British touring in Spain.

If Spanish. All trailers over a very small weight and with it's own brakes (and all caravans are over this weight and have their own brakes) have to be registered separately. Sadly I dont know the way forward
on that score.
Will the people selling it deliver it for you for a small price?
Hope you get it sorted.

Nicson.