How much money ya got?
By far, the most common wood used is pine. Then you'll find (usually) 3 grades of pine bee gear. The cheapest may have cracks, loose knots and a certain amount of allowed warping. Some commercial outfits use this to keep expenses low. It gets painted anyway, right?.
Next will be straight boards that could have a few tight knots. I use this, because an occasional tight knot doesn't bother me and it helps keep expenses down.
Next will be straight, clear lumber. Some people insist on this, though it will be painted. Expense is higher.
You can also find cypress (very weather resistant), cedar (also weather resistant), various hardwoods, &c. Can be very expensive, unless you mill your own.
Then you have those who build their equipment out of scrap lumber, or lumber they mill themselves, with great results. Very low cash outlay, but lots of work.
That's if you're assembling your own gear. If you want it preassembled, you will pay even more for it.