Hello timebandit,
300 - 350 pounds sterling is roughly 380 - 440 euros today, it is possible on the Costa Blanca south, you would easily get a long term rental for this price.
The only thing is because of the season just coming in, many do short holiday rentals as they profit more from them - but you should be able to find someone willing to let it out long term to you.
When are you thinking of arriving?
What are your long term plans? Are you seeking work out here?
We can put you in touch with a couple we know of that we send people to, they have many privately owned apartments within this price range.
Let us know a little more of what your plans are, knowing where you want to be would help a bit more as Spain is a big old place
