I’m going to go down the 10mm route too when I start my ACW force too. I started wargaming with my dad using his Airfix Union army in the late ’70’s so there’s always that nostalgic pull for me – the rules we used sound a lot like the ones you are talking about, not sure but were they by the London Wargames Group or something similar?
For painting tips have a look at the Bend Sinister/Redline Miniatures site, there’s a how to on there – I’ve a few Napoleonics in 10mm and they are a breeze to paint (especially if you aren’t going for every button etc.) so I’m thinking that the simpler ACW uniforms will be nice and quick too. When I’ve painted butternut before I’ve used differing shades of lightish to earthy brown (if that makes sense) even a greeny-grey might be ok – have some fun mixing them up! I’d check the Nafziger Collection out for orbats etc.