can i successfully plant a hortensia in Spain, Costa Blanca

I'm not sure - this is a plant of temperate climes, not sub tropical. You could try, if you keep it very well watered and in a shady spot.

At least you won't suffer from late frosts nipping the new shoots. Keep in moist and shaded. Water well after planting.

Do you mean hydrangea? We had a gardening 'expert' inflicted on us in Italy by relatives and he planted a load of them - both the green-leaf mophead type plus the silver-leaf paniculata. Both are a disaster and not worth the space they take up. They need copious amounts of water, particularly the greener types, and only produced flowers last year after daily watering during the hot season. This year I gave up - despite the fact we have irrigation, they simply do not receive enough water and I feel that to water them everyday is just not very ecological. The silvery/white leafed varieties will grow up, but foliage only - the flowers are not worth waiting for. The green mop-head types just get burned by the summer sun.

Where we are in Italy, we are talking about summer temps of 36-40C. If your climate is similar, I personally would not bother. You may have success, but you would need so much water to do this, it seems a dreadful waste.