Started a new Militarum Tempestus army as well, recently.
Imho, the Start Collecting! set is so ridiculously good, that I bought 4 of them. Now I have all the infantry I want and 4 Tauroxes (which is probably not as many as I want).
My advice would be - do not mistake Scions for just "a bit another Imperial Guard". They are not. They are more like imperial version of Dark Eldar and the best army for them, imho, will be a crazy MSU. With that in mind, you should think of what do you want your army to look like.
I have that easy, intending to use my Scions as allies to my Baronial Court. If I were not, I would buy more Tauroxes (like 7 in total + a single Imperial Knight, which really, really helps to fill the gaps in the strategy).
On small games, I would probably use the Hellrain Brigade from the Start Collecting! set (so Comissar, Command Squad, regular Squad, one Taurox), would buy dedicated transports for both the squads (so Taurox 3x total) and add Inquisition - Inquisitor, Henchman band (shooty or CC, not sure yet), a flyer (Valkyra or Stormraven? The first is more stylish, the second is better) and hide the Comissar into the flyer, so that the Scions could respawn.