As ZetaTre reported in post #9, diesel-filters.com is a good supply source. The check valve can be found by using tabs to "Browse By," then "Manufacturer," then to "Mann Filter." The check valve is sku:2400843621, or
a direct link is here.
I don't know how much sump pressure should be expected. The best discussion of sump pressure and check valve placement that I found is between 3:00 and 3:30 on
this YouTube video. Not a whole lot of detail, but that is the best I could find.
Hope that helps.
Stephen