I asked a similar question a few days ago

I did give a response to /u/J2quared's post last week that covers much of the relevant ground. Also, some relevant information can be found in my last two posts:

[On the clubs and militias that sprung up around Germany in 1918] (https://www.reddit.com/r/AskHistorians/comments/4gqtjr/what_was_the_significance_of_the_new_political/)

On the German military's involvement in Hitler's rise to power

In Bavaria especially, but throughout the Reich as well, there were numerous organizations that were in effect only very slightly different than the Nazis - former front soldiers, feeling humiliated by defeat, unable to acclimate to civilian life, usually racist/antisemitic, and often associated with the various Freikorps or civilian militias. Though competitors in a certain sense, they were also natural candidates for alliances with the Nazis. Ian Kershaw gives an excellent example of the type. In 1922, with the Nazi party at only 2,000 members, Hitler forged an alliance with Julius Streicher, the infamous Jew baiter of Franconia who would be sentenced to death at Nuremberg after the war. Streicher had been a member of the German-Socialist party (the name itself gives you a sense of how unoriginal the idea of "national socialism" was) and brought some 20,000 followers with him to Hitler [Kershaw, 178-179].

The Streicher example shows that many joined the Nazi party by following another leader. After World War One, many dissatisfied or nationalist or racist men sought membership in various local clubs and groups that each subscribed to the "fuhrer principle," which was itself an innovation of the youth movement of the pre-war period [Waite, 50-51]. When these various sub-fuhrers found places in the Nazi movement they brought their followers with them. The early Freikorp men first followed leaders like Paul Ehrhardt or Ernst Rohm, then followed them to the SA; similarly, when Franz Seldte formally gave his allegiance to the Nazi party in 1933, the Stalhelm followed him.

As for other specific clubs, there are practically an endless number representing an array of interests, united in their hatred of democracy. There were Pan-Germans, monarchists, the DNVP, the Shutz-und-Trutzbund militias, and the Einwonerwehr civilian defense units. There were also groups for intellectuals, like Moeller van Den Bruck's influential Herrenklub. Many of these groups joined the Harzburg front alliance of right-wing parties in 1931, nominally meant to oppose the Bruning government, but implicitly an attempt by Alfred Hugenberg and the DNVP leadership to co-opt Nazism. But the Nazi leadership was adroit at peeling off these conservative interest groups, as with the Rural League, a coalition of farmers once controlled by landed proprietors of eastern Germany that the Nazis courted away from the DNVP in 1930 [Craig, 552].

So, as with Streicher's wing of the German-Nationalists, Ehrhardt's Freikorps, or the Rural League, you're really looking at a constellation of clubs, local, regional and national, all sharing an anti-liberal outlook, that slowly funneled up into the Nazi movement. Probably few people ever faced a compelling decision to join the Nazi party versus some other party, but were recruited in an opportunistic way into some militia or interest group that later went over wholesale to the Nazis. That excludes of course the many millions who joined once it became expedient and advantageous to do so.

As discussed elsewhere many of the militias and clubs, certainly including the Stahlhelm, received a healthy amount of funding from wealthy industrialists, and a good deal of arms from the Reichswehr. (That is by way of quickly addressing your first question - you gave me a lot to cover!)

On the other hand, allegiance to some other special interest might still be compelling reason NOT to join the Nazi party, but Hitler was extremely clever about co-opting these hold-out special interests, as with the Rural League. As one example, when his Vice Chancellor Franz von Papen negotiated a concordat with the Vatican in 1933, he was essentially undercutting his own Catholic Centre Party on Hitler's behalf. The agreement with the Vatican in place, many former Catholic Centre voters could feel good about joining the NSDAP [Craig, 582]. Those groups on the left that held out longer were eventually neutralized or "coordinated," as when the Nazis arrested the labor union leaders and abolished their unions in May 1933.

Note: There's much more I could add, but I'm hard-pressed for time at the moment. Would love to give you more information if you have a follow up question, and thanks for the tag!

Cited:

Gordon Craig. Germany, 1866-1945. New York, NY: Oxford University Press, 1978

Ian Kershaw. Hitler, 1889-1936 Hubris. New York, NY: W.W. Norton and Company, 1998.

Robert G.L. Waite. Vanguard of Nazism, the Free Corps Movement in Postwar Germany, 1918-1923. New York, NY: W.W. Norton and Company, 1969.