One of the only ways currency would work well with the current things implemented into SMP is through a server mod. You'd use commands to exchange money instead of trying to have people term different physical resources as the official currency. This would let funds accumulate in a treasury every day, where admins in "government" positions can withdraw and distribute funds to players starting up shops. The the shop owners can use their funds to pay workers to collect materials for them etc. "Banks" could give out loans to players just starting out, and after they buy some tools and work for a bit, they could pay the bank back and such.
Personally I'm a fan of just swapping physical goods ("I have a lot of iron but need gold, anyone want to trade?"), but the above would be decent if you really wanted to have a currency system. It'd probably be hard to start from scratch, but all economies are unless slapped with a big starter pack of cash.