Check out Steven Thomas's website link A bit dated now in places but a really good start online for the wargamer.
In brief, I'd suggest your first decision is whether you want to play with 'generic' SCW armies or those based on a specific timeframe battle and/or theatre. It's a common but mistaken conceit to lump all the elements of the war into a single melting pot and treat the forces of 1938/9 the same as those in 1936.
At the start of the conflict in 1936, you have mobile columns of political and trade union militia with hodgepodge support from loyal Asaltos and Peninsular troops, homemade armoured vehicles, the first international volunteers and a few Guardia Civil of dubious loyalty.
Facing these were rebel Peninsular army troops, Guardia Civil, Carlist Requete and Fascist Falange volunteers in their distinctive dress, though at first this was quite simple.
Very different professionalised armies faced each other from 1937 onwards.
Your choice may be dictated by your personal political leanings or culutural references such as Orwell's Homage to Catalonia. Maybe you just want 'colourful' armies or something a bit more fun than lines of khaki tanks and machineguns and damn the politics.
It may well be that if you have or have a hankering for WW2 Italian, early WW2 Russian, or Blitzkrieg Germans as much of the heavy weapons can crossover. Build a 1938 Italian
force for the SCW and you can use it in the Western desert with a few changes.
Choice may be down to figure availability in a particular scale, or your preference for a certain manufacturers figures.
My own 15mm Spanish Civil War armies allow me to fight the early 'Columns' period, where enthusiasm was more important than tactics, and where you can mix and match regular Peninsular Army and Para-Military formations to your hearts content with the volunteer and political militas on each side. Fun to collect, paint and game.
Added to these I have more regular forces based on the Battle of Guadalajara – Italian CTV and XII International 'Garibaldi' Brigade as I loved the idea of a fascist vs communist Italian grudge match.
Painting the figures, I suggest more greens in the Nationalists and more browns in the Republican forces (Vallejo Brown Violet is good). More Blues, blacks and civilian clothing early on. It is easy to mix and match figure bases if you choose your palette and troop choices carefully, creating different units wit character without having to go to the expense of buying all new forces.
For instance my Garibaldis are given a couple of bases of guys in black berets and typical checked Basque jackets to become a Basque formation. Add in a few more Pasamontanas and I have 'Connolly Column' (British Battalion).
Other pointers? Gaz trucks are good all rounders, they were based on a Ford chassis and suitably 1930's generic. Artillery? Anything Italian/Austrian WW1 or the larger French WW2 items (105 and 155mm) is rarely wrong – I go for the look of the field piece rather than strict accuracy. if in doubt, slap on spokes rather than pneumatic tyres. Please don't get French 75s or German 77mm – too hackneyed by half. Oh and if you can, base your gunners separately so you can switch the guns between armies or even use some of them in other conflicts (eg WW1/WW2).
Treat Bueno's colour uniform plates with due caution unless you want to play pulp Spanish Civil War.
Rules? You are on yer own with that one. A subject all on it's own.
Cheers
Mark
finds bases at the dollar store!
