Use your Strike Squads as Strike Squads. Specific loadout doesn't really matter much but keep them cheap and avoid the special weapons (sadly, this is going to be the case for most things).
The Terminators should be done as Paladins, probably with a mix of falchions, halberds, or hammers. Again, kept cheap where possible.
I'd say your next purchases should be:
Dreadknight - build as Grand Master (just give him fancy gubbins)
Stormraven - give it the lascannon, multimelta, and hurricane bolters
imo
more Razorbacks - lascannons or assault cannons
From my experience with the Grey Knight community in 8th so far, this is the way to do things.
(Well, the real way would be 3-5 Grand Master Dreadknights and add them as a detachment to another army but I wanted to be helpful
)