There's this baptism on FMP Kent, Canterbury Archdeaconry Baptisms:
St Mary's, Walmer, Kent
31 Mar 1805 Susan the dau of Edward and Sarah Brown
Here's a link to the image:
https://search.findmypast.co.uk/reco...ANT/B/96426238
There's also this one:
St Martin's, Great Mongeham, Kent
07 Mar 1807 James the son of Edward and Sarah Brown
There are other baptisms in Kent for children of Edward and Sarah Brown(e), but as those names are fairly common you might need additional evidence to prove they are the same family.
I think this marriage is very likely to be the above couple:
13 Oct 1804 St Mary the Virgin, Sandwich, Kent
Edward Brown of the parish of Northbourne and Sarah Pain OTP were married by banns
Both made their marks. Witnesses Richard Brown (mark) and John Perkins (signed, may be a serial witness)
The banns were read at Northbourne (which is 3 miles from Walmer and one mile from Great Mongeham) and as well as Sandwich.
https://search.findmypast.co.uk/reco...T/M/94043330/1
You said in your opening post that you thought Susan's mother was Sarah. Was this from a census record?
Perhaps this one? (In 1841 there was a child surname Church in with what may well be the same Sarah Brown)
https://www.ancestry.co.uk/interacti...cklabel=Return
This baptism looks very promising....
St Mary the Virgin, Ripple, Kent
09 Aug 1778 Sarah the dau of Finnis and Elizabeth Payne
and a possible marriage for her parents:
13 Oct 1777 Rodmersham, Kent
Finnis Payne m Eliza. Soames
I have run out of time to type out in full. Rodmersham isn't near the other places, but the names were a good fit, so might need a bit more checking out!