I, personally, try to get beginners on the double horns ASAP. Once you are at G# second line and higher, and F 3 lines below and lower, you're playing on the Bb side of the horn. While the F side fingerings are nice to know for those notes...99.9% of the time you're using the Bb side. Why teach them and get them used to the F side fingerings on a single horn when they are going to have to learn the fingerings all over again (Bb side) once they get a double in their hands?

As for the horns themselves, my personal preferences are:

Conn 6D
Yamaha 567

Like I said, these are my personal preferences...other people will definitely mention other horns that will be good options as well. If money is an issue, try for the 6Ds as there are going to be more good used horns out there than the 567 in my experience. The 6D is also less prone to "kid damage" than the 567.