Terrassa is in a lovely area not too far from Barcelona. The town itself is an industrial town but has 3 lovely churches in the centre and hosts a jazz festival every summer.
Now, upping sticks altogether. My view, for what it's worth, is that you should go for it in rented accommodation. I think this area may be a bit more expensive to rent or buy than most of us are used to on the Costas, purely because there is not as much available. You may be better off looking at rentals on the outskirts of Barcelona and commuting. Trains take about 40 minutes from Barcelona.
Again, IMHO, selling up in UK and moving over lock, stock and barrel, may not be such a good idea as you might find it a/ difficult to get a good price in UK at the moment, and b/ you would have to get back on the market in UK if it didn't work out. I would suggest that you would need enough behind you to exist for 6 months (rent, living expenses) to make it viable. It would, however, be a great shame to miss out on this opportunity this presents for yourself and your family. A 3 year old would be ideal as they would not yet be in the education system but would be able to start mixing in and have a great start when they eventually start school. The education system in Spain is excellent and they would have a great start in life with the added bonus of probably becoming bilingual from an early age.
Remember that they will need a different immunisation schedule to start school in Spain as opposed to UK, for example, Hep B vaccines are required in Spain which aren't normally given in UK as a childhood immunisation.
If you can a/ afford it and b/ bear missing the kids (and missus) for extended periods, go over and rent until you have settled in. A bit later, you may find they can join you and you could rent out your house in UK to cover the mortgage payments (make sure you get a good agent who will look after the property for you).
Worst case scenario is that you will refuse the job because of the financial aspects and, in years to come, regret the fact that you didn't take the chance when you could.
I moved overseas with my family when my first daughter was 1 year old and came back to UK when she was 18. (Not all in the same country, I hasten to add). My youngest daughter was born in Cyprus and she was 15 when we came back to UK. I was in the fortunate position of having a steady income all that time but I am so glad that we did it. My only regrets are that both kids went to non-local schools so didn't get to pick up the languages as they mixed with British kids only. That, looking back, would be one thing I would change.
I have now been back in UK permanently for 15 years after spending 25 years abroad. I am now looking to early retirement and moving to Spain as I am rather disillusioned with the UK and look on my time overseas as giving me a far better quality of life, plus making more lasting friendships, than I am getting in the UK.
It's your life and only you can make up your mind on what you want to do. I don't know how old you are but, if you have a job waiting for you and you like the area, go for it and see how it pans out. I do, however, think you shouldn't burn your bridges and should have a bit of cash to fall back on as Spain is not renowned for it's generosity on the wages front. But what an opportunity to grasp if you can.