After years of being on the fence I finally went and bought a Start Collecting Seraphon box. Where should I go and what should I do next? I've heard Slann are good but what about the range for my army will I have enough if any ways to deal with further threats or should I just try to close distance as fast as possible?
You've heard wrong, slann are not good! We have far better units in our army, slann is ok, but expensive for the points. I'd try and build up a shadowstrike force, one of our most powerful battalions and one of the easiest to build.
With the start collecting box, my advice os always build the carno, use the skink oracle from that kit as a starpriest, take a scar-vet head from carno kit and convert one of your knights to a scar-vet on cold one. A @scubrat said get some skinks and chameleon skinks, and ripperdactyls or terradons. Bastiladons are good to get for anyone collecting, and a starseer for a good wizard. A second start collecting is a great way to go as well. Slann and engine would come later as you want to expand.