We lived in Chalkwell for 14 years, and loved it there. The boys went to Chalkwell Hall School, and were happy. It's got a pretty good reputation as a school. From what other friends have said, Leigh North Street is a very good school, and would be my choice now. West Leigh schools have a reputation of being academically pushy - some parents like this, others don't. They have (or at least had - it may not be there any more) a swimming pool, but the playground is right by the A13, at a set of traffic lights, so the air quality may not be that special.
As far as senior schools are concerned, Essex has the 11+, so you can choose for your child to sit the exam and, if they pass high enough, they can be offered a place at a selective school - in Southend that would be either Southend High School for Girls, Southend High School for Boys, Westcliff High School for Girls and Westcliff High School for boys. All of these are very good schools with excellent reputations.
However, some people believe that the fact that a lot of the best pupils are creamed off by the selective schools does have an impact on the other comprehensives in the area - I'm not sure. Ds3 would have been going to Belfairs school, had we stayed in Southend, and we were happy about this - we felt he'd be happy there and would be encouraged to do his best.
You can save quite a lot of money by going slightly north of the London Road - we did, and were very happy in both the houses we owned during the 14 years. If either of you are going to be commuting to London, there's a regular and frequent service on the c2c line, and you can find property within walking distance of Leigh, Chalkwell or Westcliff stations.
Leigh is the most expensive area - there are some lovely houses, and the Broadway is a nice shopping street with some good restaurants on it, as well as a library and a little park.
I hope that this helps - please feel free to ask me any other questions. I loved living in Chalkwell, and made some wonderful friends there - and I miss both Southend and my friends now that I have moved to Scotland.