Welcome to AoS.
I am sure you have read around enough to know that Slann, Saurus Astrolith Bearer, and Engine of the Gods are pretty much mandatory. However, in escalation league I would focus more on fast attack and horde options.
Skinks are a great first buy addition to your army. They will be your go to summon fodder at higher points games and they will be a good battleline and objective holder in your smaller leagues. Long term you will likely want 4 of so boxes of them. For the time being I recommend 2. Don't underestimate the value of 40 Skinks.
Razordons are probably the best unit for Seraphon in the 1k and lower range. They are also still very strong in higher MPP games as well. Think of them as flamers from 40k. They are medium ranged, high volume of shots (sadly not auto hitting), and with some rend.
If you want some big dinos for your early games then I would go with the Scar Veteran on Carnosaur. The other dinos are too expensive of too fickle to be viable in low point armies.
Best of luck. I hope you enjoy your time in AoS.