I live in upminster and use this line frequently.
The train from fenchurch street runs along the coast once you're past basildon, and ends up near southend shopping centre, a 5-10min walk to the sea
if you take the Liv street line, that runs via brentwwod, billericay etc, and ends near the art college. slightly longer walk. would take 10-15mins to get to the front with the arcades etc.
If there's kids with you, then use Fen St - more scenic journey - fishing boats, coastline etc, otherwise, use whichever ones easier for you to get to in London
journey time should be an hour I think.