Hello OP
My DS went to The Coopers Company and Coborn secondary school in Upminster (we live in a different town, he was very lucky to get one of the places offered to out-of-catchment kids via a lottery). One of his closest friends lives in Upminster, so I'm happy to share what I know.
The entire town is lovely! It's the last stop on the District line into London and also has a very regular fast overground service into London Fenchurch Street. If you go in the other direction, it's about an hour to Southend-on-Sea.
The houses in the Coopers catchment area are more expensive. It's an excellent school, very focused on results. Discipline standards are high.
When my DS left, there were plans to put a new build estate in Hall Lane on the mini golf course. There were lots of objections, I don’t know if it went ahead. Apart from that, there has been very little development since the 50's and 60's. Most homes are either Edwardian or 1930's.
My DS's friend lived in Asburnham Gardens which is a 5 min walk from the station. The houses on that road are huge!
The Corbets Tey end of Upminster is also lovely. You might want to avoid the roads close to the crematorium though.
The cheapest part of the area is Cranham. IMHO it's still really nice! The whole area has lots of parks/open spaces and a golf course.
The Upminster Park Estate (Cranham end) is known locally as the Rivers Estate because all the roads have -you've guessed it- named after rivers. It's most definitely not what you'd think of as an estate. The area is in the catchment for Hall Mead, which is also a good school.
The town centre is more of a long strip of shops. There is an M&S (food) and a Waitrose. It's very easy to get to Lakeside shopping centre either by train (2 stops) or on the 370 bus.
I hope that helps.