In the Edo period, at least to my knowledge, ronin would often become mercenaries or get involved in petty crime, among other things. How was this different prior to the Tokugawa Shogunate, i.e. in the Sengoku period?
What kind of possessions would a ronin, Sengoku or Edo, have? Would he have owned a horse or a suit of armour (in the Sengoku period)?
There were no Ronin in the Sengoku period. The samurai didn't even exist during the Sengoku period; they only came into existence when the Shogunate codified the four classes of society: Shi (samurai/gentlemen), No (farmers), Ko (artisans), and Sho (merchants).
Ronin were literally a creation of the Shogunate; they didn't exist prior. The word "ronin" means "wave man" and it indicated "their free floating status in the Tokugawa order", according to Denis Gainty. Some ronin left the service of their lords to seek better financial opportunity but many were created by the Shogunate itself after the Shogunate confiscated the lands of lords who had opposed the Tokugawa Family at the Battle of Sekigahara.