Really nice song, played and sung well too. Don't think it needs any more verses really unless you come up with more lyric that you just can't resist using. A folk song like this can stand more length I think, but it isn't necessary.
There's always a temptation to add more instruments just because they're there but I like it as it is. (I'm sure you'll experiment anyway!)
Don't know about brass, but a big recorder (treble, I think - not one of those little things that everyone played at school) has a nice sound that would be OK.
Good work!