Clearly the airframe. How many companies in the World that can provide high power engines , large airframes modification for several tonnes antenna in the World.? Ever thought Why A-50 exist in PLAAF but they can produce hundreds of turboprop aircrafts of there own. U have to wake Radar is not big deal. It is the large military specification grade aircraft thats what matter.
Airframe certification was done along with new engines and extended range/MTOW. As i said when India negotiate this contract semiconductors were not built. It takes a decade usually for India to negotiate the contract.
If you feel that the airframe is the important aspect of an AWAC or AEW/AEW&C system, then it would seem you lack a fundamental grasp of the aircraft modification industry.
There are a number of different companies which produce (or have produced) aircraft that can be converted into an airborne surveillance platform, be it as part of an AEW or MPA system.
The IL-76 airliner airframe was adapted into the A-50 Mainstay.
The Boeing E-3 Sentry was an adaptation of the Boeing 707 airframe.
The E-767 is another Boeing adaptation, this time of the Boeing 767.
The EMB-145 (R-99A) is an adapted Embraer ERJ-145 with the Erieye radar.
One version of the Phalcon AEW was built onto a Boeing 707 for Chile.
A Hercules version the EC-130H was used by the USCG with an APS-145 IIRC.
There is of course the E-2 Hawkeye (AWAC mounted on turboprop...)
The Saab 340 (AEW&C) which mounts the Erieye onto the Saab 340 prop airliner...
The forthcoming Boeing E-737 Wedgetail based on the Boeing 737-700 (800?)
This list, which does not include airframes of similar capacity or even delve into MPA aircraft, which frequently have similar requirements, should indicate that there is significant choice for aircraft to carry radar arrays used for AEW.
What is important in terms of airframe/platform, is the needed range/persistance on station, as well as the ability to carry and generate the needed power for the radar array. One thing which I believe has begun to be examined is carring some of these arrays aboard high altitude unmanned airships, so that an array could potentially remain on-station for days at a stretch.
Another argument (or rather, proof) against the importance of airframe vs. radar, is that the E-2A Hawkeye prototype first flew October 21st, 1960. An airframe from 48 years ago is today really not very advanced or special, a number of different companies, in different countries around the world, could build the same or better. The radar array, and the supporting mission systems however, is a different story.
-Cheers