Based on the couple of pics you posted so far, I would be really careful putting much money into that bike. I would definitely not be spending bucks on wheels, etc.
As far as shifters, here in the states I can buy 6 speed Shimano indexed thumb shifters, complete with cables, for $10. Six speed stuff is readily available. When you want to upgrade to more speeds, I would toss this bike and find another one that already has what you want on it.