I am a fan of choosing hymns that are singable by the Congregation. I am a fan of choosing appropriate transpositions if the key in the hymnal is not in a singable range.
You must be careful, however, that when transposing, you don’t just take the upper notes into account, but the low ones as well. If your congregation can’t hit the fourth space E, and you take it down so that E becomes a C, then you need to check the lower notes too. If the lowest note in the song is a B below middle C, this transposition now asks the singers to hit the G below middle C, which is out of the tessatura for a lot of sopranos and (transposed an octave) tenors.
Composers often write in certain keys for specific reasons. The key of D Major has been historically referred to as the “Sun Key” because it has such a warm and bright sound. If the hymn is composed in a key because they wish to use the specific qualities of that key, I would think twice about moving it. If a hymn is in Eb, I probably wouldn’t transpose it down to D. The same with a hymn in F. I wouldn’t lower it to E. E is another bright key, where F can be more mellow sounding. I would lower F to Eb, and go the full Major Second in transposing it.
This is all made easier, of course, by using an electronic organ that has a transposer on it. Or by obtaining an older copy of the Organ Accompaniement Book from Oregon Catholic Press. While some may find their contemporary hymns to be tasteless, their removal of older language (thee and thou) annoying, and their gender equity reworking absurd, at least the older OAB came with a lower key option for most hymns.