An eagle eyed visitor has noticed that there’s an anomaly with off-peak fares from Brentwood (Z9) to Zone 1. The off-peak single fare for Brentwood to either Liverpool Street NR or LU is £5.50 but to any other Zone 1 LU station it is £5.40. This also includes passing through zone 1 and escaping the other side.
With Oyster you need to note that if you exit at Liverpool Street NR and then go down to the Underground you will be overcharged because the Oyster system won’t refund a fare once charged. To avoid this, consider changing at Stratford onto the Central or Jubilee lines. Contactless will work properly, as will return journeys to Brentwood.
If you want Liverpool Street itself then consider either Aldgate or Moorgate as alternative nearby stations.