If you want at least semi-competitive army, Slann is the first thing you need to consider. Seraphon units stats are mediocre at best, so it has to be compensated by constant summoning. Slann also has some other nice rules, his command ability can be very useful in certain cases and constellations are always handy.
If you assembled your flyers as 6 ripperdactyl riders, you already have all units you need for shadowstrike starhost. Terradons are also OK, but rippers benifit from starhost more. I recommend to have at least 6.
After that - a box of skinks. Better two, ideally 3. Having 40 skinks is a bare minimum for seraphon player. 60 skinks is OK. 80 skinks is optimal. Thankfuly, it is not as bad as it sound. A box contains 24 skinks and cheaper than other 20-model boxes. I wouldn't bother with more than 60 at the start.
Then - get razordons. They are expensive resin kits, so consider conversion. There are multiple guides posted on Painting and Conversion forums. Dark Aelves' Drakespawn Knights are often used as a base for conversion. You'll need at least 4 razordons. Skink handlers can be easily converted from spare skinks, but they are not mandatory unless you are aming for Thunderquake Starhost battalion.
Finally, Engine of Gods. It is assembled from stegadon kit. Its abilities are very-very strong.
These units are bare minimum, which every Seraphon player should have, if he want actually win games. From here you can go there you want. Saurus-heavy builds are not impressive nowdays, and you'll need at least 60 warriors to make them work. Skinks feel much better - you can just swarm battlefield with them, supported by large beasts and summon more, when they die. You'll need 80-100 skinks here. Monster-heavy are also good. If haven't assembeled your Casnosaur yet, build it as Oldblood - stat-wise it is worse than scar-veteran, but it's command ability changes everything. Scar-veteran is still strong, but harder to pilot. You'll need to choose targets very carefully. I won't advise building trogglodon until you have both Oldblood on Carnosaur and Scar-Veteran. Oldblood on foot is not worth it for now. Bastiladons and stegadons are good in their own ways, but if you want extremely strong thunderquake starhost, two bastiladons with solar engines - they are extremely durable and can be damaging too. Ark of Sotek can be ok, but more situative. Stegadons are good in any build - Flamers are strong against hordes, and bow has very long range. It is better to either magnetize weapons or have both variants. Regardgless of the way you chose, you'll benifit from Astrolith Bearer - it is a great force multiplier.
Start collecting box is great - you basically get warriors and knights for free, but units which they contain, are not top choices for competivie play, or at least hard to use. I'd get two SC boxes to have all types of Carnosaur and trogglodon. 15 knights are useful for one of our battalions and saurus can be useful for summoning. But I won't bother with it, until you get all other stuff.
As for Kroak and Slann - Kroak needs special setup to pull its weight. His main strength lies in devastating mortal wound spells, but they are hard to cast. For a pure summoning purpose - Slann is better because of lower cost.
Finally, keep in mind, that new General Handbook edition will be out very soon. It will change costs of units and, probably, allegience abilities, which may shift internal balance for seraphon.
I hope, that wasn't too confusing.
