Dual stage will keep the pressure constant,single stage will drop as tank pressure drops.Single will work,you just have to keep an eye on your pressures,and with small bottles,that means the pressures will drop sorta fast.

I second buying from a welding supply.They will support what they sell.

I'm not sure they sell a kit with that handle,you may have to buy it alone.

Just for the record,I don't build bicycles,I do weld and braze in my automotive fabrication work.So as for tip size,I would think that they use a 2-3 to spread the heat around.Somebody will know better than I do.