Mostly fitting for high and late period, from late 14th century.
What I imagine:
Crossbow of good quality.
2 handed handed axe (good for ship)
1 handed Messer and buckler (It gives a sense of that civilian flavor)
1 handed axe and small pavise
Brigandine
Mail
Sallet for later version, Kettle hat for high period should do the trick, but it doesn't have to be uniform. They're pirats after all. Arrrhhh!
Some form of civilian flair.
Quite possibly recruited as a ship unit, even with siege weaponry, but doesn't have to be.
They seem to be made up of lowborn nobles and foreigners, as mercenaries often were. Armour accordingly (for example brigandine) They were using the smaller but mobile Longship, as well as Kogs and Holks, so you could potentially make a navel unit out of this, but that is off course up to you. Some form of cannons on the ship and report of guns, but mostly crossbows until the very end of the 15th century.
The picture from the Gotland church is fairly unclear, but it's supposedly contemporary image of the men around early 1400s. The guy on the far left seem to have some sort of helmet from Novgorod and the main guy same to have some sort of "russian hat" that you can see
here. Perhaps there is some influences from raiding/trading with Novgorood, but again I could be way off. Maybe it's just a standard Bascinet on the guy on the far left. The image seem kinda strange, so I guess it's up to you. At least there is a big axe there, possibly a civilian
messer over their head (the most sexy looking sword in my opinion

), so those could always be a type of weapon they use. I've also heard that during the 1400s their primary weapons were crossbows, swords and axes from
the German wiki article, which is quite comprehensive for a wiki article.
Their armour style
“Merchant and pirate were for a long period one and the same person. Even today mercantile morality is really nothing but a refinement of piratical morality.“ (a nice quote from Friedrich Nietzsche summerizing what we often see military commanders doing during peacetime at this time, much similar to the activities of the Scandinavian Lawless. Raid one day, trade another day. To my understanding they were mercenaries, but also merchants in between their raiding. Perhaps these men could show some characteristic of civilian gear to set them apart. A small signature here or there like a bag, hat or other civilian flair. Here are some civlian gear from 1367 to 1400 (area from Wallonia, Hesse and Thuringia):
http://imgur.com/a/qAdDJ or here
http://effigiesandbrasses.com/media/...5342_large.jpg
A modern image of the 1400 capture of the victual leader and his gang
here before their execution. (the helmets of the guards look weird, but otherwise ok I guess).
Here is a picture from Hamburgh museum of a reconstruction of their leader based on the 600 year old scull... and an axe in the background,
hint hint:
Another representation from Rügen of their leader:
http://p5.focus.de/img/bilder-des-ta...4/bdt-sand.jpg