Dacians (or Gets, depending on the source who named them) are a branch of Thracians indeed (north of Danube) and they've been in contact with the Celts, not so much around 100 AD though when the Romans conquer part of Dacia, Burebista the most important Dacian king crushed them 150 years before. The long shirt and trousers outfit (plus the hat in cooler weather) was still very common among Romanian peasants until the mid 20th century, the more stylized ones are kept for certain celebrations.
The Dacian civilization was quite advanced as the Sarmisegetusa (Decebals capitol) religious and astronomical complex (as old as 4-5th century BC) proves it or the chain of fortifications (dava citadel) they relied upon. The various Dacian tribes were rather homogenous in customs, their often quarrels were caused by their individualist and independent nature. The oldest written tablets in the world the Tartaria tablets are there, certainly much, much older than the Dacians themselves. The area is a naturally built cradle for civilizations, relatively protected, fertile, lots of rivers, rich deposits of silver and gold, easy to exploit salt a critical element for preserving the food, etc.
Regarding the soldiers. The climate seems to have been harsher than today, because often in the winter, Dacians could cross the frozen Danube (havent heard of Danube to freeze in decades) to raid into the Roman empire. The most important kings had permanent armies, Burebistas was mentioned at 200,000 men but that sounds exaggerated even if his kingdom was far larger than Decebals. Permanent armies all through the year. And the tarabostes werent the middle ages grand feudals, some of them were powerful but the vast majority were smaller nobility.
Well, Im not so much caught by the various controversies around the Dacians but there are a lot of intriguing aspects, especially some of them, regarding technology, religion and pure science are quite intriguing. A site (in English) where you can find a lot of interesting topics if you ignore the exaggerated passion there :
http://www.dacia.org/Dacian_Virtual_Museum/dacian_virtual_museum.html
A more interesting site would be :
http://www.gk.ro/sarmizegetusa but is only in Romanian and is slow (at least on my connection). Has an extensive section about the army, weapons, organization, fortifications, military symbols etc. Pics too.
From that site : FALX sabie curba de dimensiuni mari care a determinat comandamentul armatei romane sa schimbe coifurile soldatilor. Aceasta sabie printr-o lovitura data in plin trecea prin coif si ranea mortal soldatul. Pentru a contracara acest efect, romanii au fabricat casti cu doua benzi metalice rigide, dispuse in cruce cu intersectia in varful coifului, locul unde era aplicata de obicei lovitura. Numai asa s-a putut diminua efectul devastator al acestor arme. large curved sword which determined the Roman command to change the soldiers helmets. This sword when managed a full blown hit passed through the helmet and could lethally wound the soldier. To counter it, the Romans built helmets with 2 rigid metal strips meeting in cross at the helmets top., the place where usually the strike was applied. Only like that the devastating effect of this weapon could be diminished.
SICA sabia scurta caracteristica dacilor. Aceasta arma de dimensiuni cuprinse intre marimea unui pumnal si a unei sabii mici, incovoiata ascutita pe exterior, era foarte eficienta in lupta corp la corp. the short sword
characteristic to Dacians. This weapons dimensions were comprised between a dagger and a short sword, curved, the sharp edge on the exterior, was very efficient in close combat.
The bow was used with extreme efficiency and was carefully made, from selected wood.
The shield was medium large, oval-shaped. Wooden reinforced with resistant leather, in its middle had a piece of metal called umbo.
Cavalry used a heavier spear, infantry a lighter one that could be thrown. Also a battle axe was common, that one too could be thrown at the ennemy. In fact is the same with the Romans the short sword and shield was the rule and the legionnaire carried the spear too. The Dacian footman could carry an axe or a spear in addition to the sica and shield. When engaging in close combat a short sword is easy to draw and handle and allows the other arm to hold the shield, makes sense.
Tarabostes used the straight, medium sized, Celtic type sword, sometimes helmets, but apparently they disliked them, same kind of shields but more refined, body chain armor of the Roman or more often Sarmatian type.
My suggested model for the common foot soldier : fully dressed, long shirt, trousers, no cape, the hat (they used it too, without the ornaments tarabostes had) but I admit the naked head is equally logical, the SICA sword and the oval shield.