I play mello! Mello is surprisingly easy to switch to. In our section (only 6ppl) 1 is a bassoon player, one is a trumpet player, and the rest of us are horn players....
The trumpet player is amazing on mello, he can nail all the parts! He is great! He, of course, was very talented on trumpet but nonetheless proves that switching to mello from trumpet can be done rather easily.
Also trumpet players tend to have great range on both french horn and mello.
A B flat mello might be better (that what our trpt player has), and you wouldn't have to worry about learning the fingerings!
So if you're interested I think you should give it a shot!