I'll go through your questions in order, because most of them are pretty simple, and close to the kinds I've answered before about operating muskets.

what does it feel like to use one -- is there significant recoil and a large quantity of smoke? do you need to be perfectly static to fire it?

The recoil would definitely be noticeable, but not more than a modern shotgun or similar. The recoil depends on a few factors, but weight of the barrel and stock, the powder load, and the size of the ball fired. None of these were in any way standardized in the early 16th century. Nor, for that matter, was the name arqubus. Hackbutt, Hackebus, Firelock, and about a dozen other names were in use for describing this or similar weapons. We do know that arebuses, generally, were heavy enough that they were often steadied by a rest placed on the ground. The barrel being so heavy, the recoil would be less severe. Smoke would always be a factor, though, as black powder was a dirty material, and voluminous white smoke was a problem until the standardized adoption of smokeless powder in the late 19th century.

Given that you'd be carrying a stand, firing stationary would be pretty much assumed, but there was a huge number of firing and reloading procedures - again, militaries weren't standardized, and much of the early doctrine of firearm usage in the pike and shot era was experimental; early military treatises were often just reprints and translations of ancient military tracts like Vegetius, adapted and updated for contemporary usage.

how is the gunpowder and shot typically carried and stored (on an individual)?

Again this isn't really standardized, and different regions and times might have different answers to this problem, but generally, soldiers made use of powder bottles - usually separate ones for the coarse grain powder that went down the barrel and a finer grade of powder for the flash pan and priming. By the late 16th century and by the time De Gheyn wrote his treatise, it was standard for soldiers to wear a belt over their shoulders that hung individual, pre-loaded powder bottles, and a separate bullet pouch and priming bottle. The belt was commonly called "the twelve apostles" because it usually carried twelve individual charges. But powder flasks with coarse powder would not have been uncommon.

how is it loaded and fired? maintained? what do you need to do before you can actually fire it, if anything?

Jacob De Gheyn's The Exercise of Armes illustrated more than thirty individual steps for loading, priming, and firing arqubuses and muskets. In general, the powder charge (from a bottle) would go down the barrel, a ball from the bullet bag dropped in on top, and the whole thing would be rammed down along with a patch to keep the ball and powder in place. The flash pan would be primed from the finer powder flask, and then covered by a pan cover would slide over top to protect it from weather and from being scattered by movement. The match cord would be held in the left hand, loaded into the jaws of the serpentine prior to firing, and when the trigger (or, as was also common, the firing lever, like a crossbow's lever) was pulled, the serpentine would drop down toward the flashpan, where the pre-lit matchcord would ignite the powder in the flashpan.

Maintenance was fairly simple. Grease or oil would be used internally and externally to prevent rust or corrosion, and after firing, hot water would be used to scour out the powder remnant - a viscous residue called fouling. Wet or oiled rags would finish the cleaning, and protective oil would go down the barrel and along the barrel outside, if needed.

how long does it take to load and fire? is it reasonable to expect a load and fire time of fewer than two minutes in the hands of an experienced arquebusier, or not?

Two minutes would be an extremely long time to fire. De Gheyn's manual exercise specified a large number of steps, but many of them are articulations of half steps that can be performed quite quickly. While experienced soldiers in state armies of the 18th and 19th centuries could be expected to fire one shot every twenty seconds, at a conservative estimate a loading and firing speed of thirty seconds or so wouldn't have been unheard of.

how does it perform at close to medium range, can it punch through a helmet?

18th century muskets were considered accurate at 80 yards, and there's not too much different about the construction of early arqubuses that would have made much of a difference. Even in the 18th century, poorly bored or badly made muskets were not unheard of; it likely would have been a larger issue in the early 16th century. However, whether it would go through armor or helmets depended more on the helmet than the arquebus. A large number of armors in the early 16th century were "munitions grade," basically mass-produced and roughly finished harnesses that were likely not heat treated. A shot from an arquebus would have a pretty good chance of punching through a breastplate or helmet that hadn't been heat treated.

So, something to keep in mind thats a quirk of early matchlocks was that the serpentine was often operated like a lever; that is, pulling the trigger was a fairly simple mechanical process in which the serpentine would drop to the same degree that the trigger was pulled. Unlike a later flintlock (or other mechanical lock), there was no snapping spring or trigger release. That means that you never quite know exactly when the match will ignite the powder. Furthermore, if the match is too long past the jaws of the serpentine it might miss the flashpan entirely. They're quirky weapons that require a lot of experience to use with reliability, though the operation is superficially simple.

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The loading and fireing operation can be found in Jacob De Gheyn's The Exercise of Armes.

If you can read German, Jakob Wallhausen and Leonhard Fronsperger wrote military manuals - taking a lead from Vegetius - in the mid to late 16th centuries, but again, it's unlikely that there was anything like a standardized system in place in the early part of century. Any recognizable system would likely have been a product of local or regional best practices, with a deal of variability.

For armor, you might want to look into Alan Williams' The Knight and the Blast Furnace, which goes into some detail about the hardening process in common use for both munitions grade armors and higher quality harnesses meant for wealthier combatants. Ewart Oakeshott's European Weapons and Armor also explores 16th century armors that would be common in your period.

I'm also happy to answer follow-ups.