If however, you have trumpet/cornet converts, they will be comfortable with Bb parts since the fingerings will be the same as their trumpet/cornet.
I know this is confusing to non-horn folks. As to new replacements for your old instruments I would stick with Bb marching horns since your students are familiar with them.
We could get into a big discussion about mellophones(F instruments) verses Bb marching horns. Mellophones project better but are more difficult to play in tune. Bb marching horns have a darker tone and will blend better with your band.
I have inherited both types and prefer Bb marching horns. My bands that have made the state finals were totally Bb horn outfits.
You asked about the range of Bb verses F horns. The F mellophones have a slight advantage on the top end but the trade off in poorer intonation is a deal breaker with me.