Jake, First thing is to research Marston Sunbeam and the later BSA Sunbeam. Originally Sunbeam were made by hand (most frames were) then like most bikes, gradually deteriorated as the market changed.
A Sunbeam roadster in 1920 could cost £22, by 1935 about £8.
A BSA roadster in 1920 was a little cheaper, and by mid 1930's about £6.
The right Subeam may well be collectible, but quality, collectible and value are not compatible.
In the early 70's I had a 1931 Marston Sunbeam 3.5 litre car, which could be compared to Rolls Royce, and like many companies, Marston collapsed around 1930, when the market wanted the £100 car, not £1000.