I guess you can well start with the cheap case, just don't start with going under 30 meters
10 - 20 meters should be fine, if the case is supposed to be ok for 40 meters.
Just that you know, every meter you go deper, the less light you have. Below 15 meters, in clear water, you already have to crank up the ISO very high to have enough light to picture the moving animals, corals, ...
Regarding the filters, once you have a flash (or very strong LED torch) you don't need them anymore, as you have more or less full spectrum light which gives you the correct colors.
The S100 can make RAW images. Thanks to this you can fairly easely adjust the colors afterwards in your favorite image processing program. Otherwise put in the "underwater" mode, which also corrects the colors somewhat.
But as the camera doesn't know how deep you are, it can't really fix the color 100% right, same with a filter.
In any case, you safe all those problems (filters, after editing, high iso, unsharp images because of movement,...) with one flash. I own a Sea&Sea YS-01 (would buy the YS-D1 today).