Hi Saxgirl,
You're setting yourself up for some work! I grew up listening to my brother practising the french horn. It's a very, very demanding instrument. You know how they say that all instruments are difficult to play well? The horn is one of those instruments that is easy to play very badly too - like the violin, I guess.
I'm a few months into learning the bass trombone and my progress with it has been reasonable - but I tried to play my brother's double horn the other day, and the mouthpiece is terrifyingly small compared to a trombone, and I made a very unrewarding sound on it. I would never dream of trying to double on horn - I'm in my early 40s now and I don't have enough time left to make it sound any good.;} Anyway. I'm mainly a jazzer and with the notable exception of Julius Watkins it is a pretty useless jazz instrument. Very straight-laced sound, amazing in Mahler symphonies, clunky and out-of-place in "vernacular" music.