If you're wondering what would be the best tradeoffs, If you're in a heavily forested area, then produce wooden spiked balls: They fetch a hell of a price apiece (even at crap-quality), and are cheap and quick to make. If you have a mass surplus of stone, and are trading with elves, produce rock/clay mugs (but not glazed ones, since wood products are used in the process, and they would be offended by them; if you can make an obsidian farm, produce obsidian mugs like no tomorrow. Mugs also produce 3 mugs-per-stone; minimum price is low, but after a few levels, they become quite trade-worthy (just remember not to trade the bin they are transferred in; unless it's non-wood or non-wood-based (like glazed clay or clear glass; green glass is okay, however, since no wood is involved in their production process. So for best trade, I suggest green-glass bins and barrels, and trade things through those; elves won't give a damn about it; and trade is much less tedious; plus, the green glass bin/barrel even fetches a decent price.)).
To maximize trade efforts and negotiations (pretty much, make the merchants want your babies each visit), and speed up your broker's levels in the relevant fields including appraisal, make a few micro-transactions at a 1:2 ratio (☼2000 sell, ☼1000 purchase (☼1000 profit), exit screen and repeat), they'll love you for it by the 5th or so transaction, and negotiations should be much better/easier in the long haul. Exit from trade, wait a few frames, and then go nuts making all kinds of crazy purchases with a 2:3 profit margin instead of less than that (they'll even counter-offer more often, just in case).
And lastly, what I suggest you purchase for the first year is some food, especially a wider variety of seeds (eating and drinking only plump helmet gets boring and gives bad thoughts), and as many typoes of items a moody craftsdwarf would possibly demand, and maybe a crate of cloth and a few balls of thread. Don't forget to grab a few cheap cut and uncut gems of various kinds while at it to cover any gem-based decorations that would also be used in the mood. Now that the bases are covered, invest in whatever else you need (I generally start with at least a bismuth bronze crossbow (makes a decent hammer when out of ammo) and some cheap metal bolts (bismuth bronze is also a cheap alternative to silver ones; copper works as well). The next year would be a more suitable time to purchase armor and such as well as all the raw materials you can get.
Speaking of which, instead of getting just plain cloth/leather bags, I suggest you purchase sand bags. They're not that much pricier, you get sand or seeds, and a free bag to go along with it. If you want more containers, get them while they're filled, especially smaller ones like bags certain barrels and bins. It's kinda like exploiting embark mode to get free bins and barrels and such just by going +1 past the containment limit of various goods.
Where dealing with Elves is concerned, invest everything in creatures first, then aboveground seeds and plants (more crop and food/booze diversity) and if they have it, invest in featherwood (or similarly lightweight wood) training weapons for danger rooms and sparring troops; it would be like they're sparring with foam pool toys or Nerf™ weapons.
Human trade investing is a combo of the two; good for filling in the blanks in the other categories like aboveground crops and such.
At least, that's how I deal with things. No pun intended.