It bears repeating that you're going to want only one way in or out of your fortress and that any exploratory mining should be done in a separate mineshaft. In fact one thing I like to do before getting serious with certain fortresses is send an expedition team that has nothing more than a proficient miner, a stone block and a copper pick and have them dig said shaft until they reach the caverns or find a source of iron.
This means I usually have them dig maybe ten z-levels down or so with a down stairs and some up/down stairs until they find something of interest. I then have the miner come back up and seal up that mineshaft in case I found anything dangerous and afterwards proceed to abandon the fortress then reclaim the settlement with the actual team that is to build the fortress from the ground up.
...I just thought it would be something useful to share. By taking a little bit of extra time you can search through a number of sites and mark the ones that you would want to play, whether it's because they have the makings for steel or they have an underground lake or anything really. If you're feeling really sadistic you can have the little drunk midget dig all the way down to the candy, preferably without disturbing the circus. That way you can have a source of candy for your more serious settlers to work with as well as perhaps a source of magma.
Then again I've never done anything with magma except drop a cavy on it from over 120 z-layers above. It bled to death on impact and exploded in a fountain of gore, so I'm probably wrong about something here.