Good work. Actually, that's worked for me sometimes too. I assumed that maybe the items got built faster when I replaced them because the newly-chosen building materials were initially closer to the site, and so were more likely to result in prompt construction, but I'm not really sure.
Good luck in your search for flux materials; I've not had much luck in finding them. If you press "z" and view the Stone menu, you can confirm which types of stone can be used to create pig iron, among other things. You should be on the lookout for limestone, dolomite, chalk, marble and calcite, apparently.
Edit: The coin economy is something that you may be pressured into when your fortress is sufficiently developed, and a certain noble arrives. The "Prices" and "Currency" menus will become available on the status screen (press "z") at this point.
You don't need to worry about it too much until then, unless perhaps you want to mint some gold, silver and copper coins (the economy seems to require equal amounts of each of these three types of coin) in advance, but basically, once the economy starts, your dwarves will be required to begin paying rent on their bedrooms and paying for any belongings they wish to acquire. You will also be able to build shops for your dwarves to use. Your dwarves will be paid wages for any jobs they do around the fortress, and you are expected to get enough coins minted to fuel this economy. Once the economy has started, your Broker will be able to tell you how many coins you should have, so don't worry about having to make estimates for it.
I've heard that some people don't like it when stacks of coins get broken up into small amounts and are left strewn about the fortress, although this hasn't bothered me yet; I believe dwarves will store their coins in any chest/coffer/container they own. In any case, the economy can be disabled if you like, by editing your init.txt file in the data\init folder.
Before the economy starts, the only real use for coins is as another trade good to use in buying things from merchants, although it's usually not necessary for that. Anyway, coins can be pretty heavy; you'd probably do better just to have some experienced craftdwarves making decent trade goods instead.
Oh, and unfortunately, merchants are greedy. They don't like to give change, so I don't think they even bother to carry any with them.
[ December 14, 2007: Message edited by: Tayrin ]