Is definitely under-powered at the moment and severely handicaps any mage forced to build into it.
The passive is
biggest problem as it makes it incredibly gold inefficient upfront, when the only reason someone is building it is for survivability. Taking 30 kills to stack up, this requires cs-ing 5 waves or 4 and a raptor camp in best case scenario which is at least 3 further minutes before the item is cost efficient, let alone making an appreciable difference.
You pay a horrendous 465g to combine
and
for just a measly 10 Armor upfront. This then becomes 25 armor and 15 AP.
The result is almost identical to buying
(+40 Armor) and
(+20 AP) for total cost of 1235g, except the latter is all up front - when you need it.
As it stands on most mages I just buy 
if struggling against AD, and later get
. Then I just upgrade to
from there (which costs 1740g, including selling the extra cloth armor).
A (simpler) alternative suggestion to your idea would be to make
build out of:
(Depending on how you wanted to tune it, I lean towards the latter) Increase the total cost of it a little, and remove the scaling passive from kills.
An additional idea would be to a new item that builds out of
and
; and it can then have QSS's active and Seeker's (former) passive perhaps tuned with more max stacks, i.e:
-
50 stacks at: 0.6 AP, and 0.4 Armor and 0.3 MR per stack;
-
Min stats: 45 AP, 35 Armor and 30 MR;
-
Max stats: 75 AP, 55 Armor, 45 MR;
-
Total cost: 2800g.
This for an alternative item for mages to build, akin to
, which emphasizes the utility early on for tough match-ups without giving you too much strength, and allowing an opponent counter-play in denying you cs to make the item reach legendary tier, but without denying you the utility and upfront survivabilty you need from the components.