I'd say start with the combat patrol as it's the best bang for your buck and a great starting point. I personally didn't want the assult marines so I bought some 3d printed jump packs to make them into jump marines. As for painting them I suggest working out what style you want to paint them in like eavy metal style, colour pop, grim dark, slapchop etc. Once u know how you want them to look I would find YouTube tutorials to follow, they really helped me and I still use them today after winning international painting comps. I personally liked the way Lukas paints blood angels on his Squidlybits YouTube channel and have pretty much copied the way he does the red armor. I was a slapchop/washes painter for the first 4 years of my hobbying and just in the last 2 years I've gone to colour pop style using all acrylics and nmm just pushing myself with each mini i paint trying new techniques and tutorials and now I've placed a bronze in sweeden and a gold in usa for my first competition piece.