Okay you'll probably want to put a pop cap at 40 or 50 so you don't have to deal with sieges before you find dat tetrahedrite.
First things first, find a sourced source of water.
Build a drainage sewer so if you ever need to flood your fort/have an accident it'll all drain out anyway.
Flood your used up gulag to make tree farms to supply bolts
Make lots of bone/wood xbows till your churning out exceptional/masterpiece(aka legend+5)[Sell crap]
Send marksdwarf squad to shoot wildlife.
You'll want full time tanners since at this fragile stage you need all the leather you can get. Look up the cheap full armor workup and make suits for your marksdwarves
Now back to the ore. From the sound of it your map will likely have a high concentration of gold, galena, and tetrahedrite. Dig down and out and you'll find it. Go find magma and get your magma forges up. you should find at least some tetrahedrite on the way.
Back to harvesting the fourth iron ore. Get your ditch, raising drawbridge, and walls up. Set up your civ alert burrow so you can withdraw safely. Then work towards putting cage traps on the inner entrance to your fort using exceptional+ mechanisms to catch any sneaky gobbos. Dig out a dodge trap(just -1 Z-level if you don't want to risk killing your own dwarves) Dig out an arena to pit gobbo soldiers after stripping them. This'll train your marksdwarves.(Thieves and animals typically escape) Forge silver warhammers. Lots of silver warhammers and lots of spiked silver balls
Load up your traps, get some hammerdwarves training to get a rough defense going. You'll want to get a milled/processed food industry going to afford ordering LOTS of ore, bars, instruments, toys, anvils, etc. Have a custom stockpile to accept crappy metal goods right by your forge. (And yes this means no just throwing "armor" stockpiles around. You need Specific stockpiles only accepting exceptional+ in your armory. Clothing goes to depot for improvement and trade, bad weapons and armor go to be melted. between the dodge trap, 20-40+ marksdwarves and cage traps you'll be fine until you find lots of tetrahedrite/melt down and buy what you need for better. I've lasted well over a decade with nothing but bone, wood and salvaged gear and you can too!