Begun the conversions yet as to model them?
As to the stats: Custodes, fluff wise, pre-date the Space Marines.
Each custodian is created uniquely, and not 'en masse' as is the case with Space Marines
or Grey Knights.
The Gene Seed to create Grey Knights is rumoured to come from the Emperor
himself, as he foresaw his demise and sought to further protect mankind with warriors
able to fight off warp-spawn.
So, which stat lines ought to b better, fluff wise?
In Iten the beauty is that individuality is fully possible.
Maybe you should take advantage of that?
Some custodes are less good, equal or better than Grey Knights.
As for armament: the Custodes armament was the inspiration
for the armament of the Grey Knights.
What do you think of these:
Custodes 1: 24518
Custodes 2: 25418
Custodes 3: 25518
Custodes 4: 25528
Custodes 5: 26518
Captain-General: 26528 (Leader)
The first two are one point weaker in either FV or SV;
the third one is equal to a Grey Knight;
The fourth is equal, save that he is one point faster;
the fifth is formidable in FV;
and the last is the Captain-General.
These stats should make Custodes playable, without tabling your opponent,
but are in keep with the Holy Fluff. A warband with five individual Custodes.
To further enhance this you could imply the rule that each Custodes is unique
and may only be fielded once per warband.
As a weapon: the Custodes Halberd:
Combat Bonus: 3
Grit Penalty: 1
The 24" obviously is for the Bolter, and to make obvious that it isn't just any
old ordinary Bolter, use the Combat Bonus and Grit Penalty of the Halberd.
These stats are a tat weaker than yours,
but they keep game-play going and are more in keep with the fluff that they are
the equals, and sometimes the better of Grey Knights.