The front Birdy hub is great b/c it makes the fold smaller and reduces the acute spoke angle for small wheels, but it isn't absolutely needed. It's still just a standard 100mm axle. Not sure what the Birdy hub weighs, but I would probably stick with that.

The problem with Velocity rims is that, for some reason, they only go down to 28h. Though I suppose you could get them undrilled and give a go at that. The Birdy rims can be bought from Black dog for US$20-something. Don has tons of them from a while back when he bought all of Burley's spare Birdy parts. To build a rear 24 spoke wheel, you need to use a 36hole hub. If you go with a lightweight rear wheel and keep the front, you probably want them to be the same size rim so you don't need to carry two spare tires when touring.