Havelsan is a system integrator and have experience with Turkiye E-7 program. But DI is not very experienced or capable at this level.
Yes that's where Havelsan come in, providing tech experience on integration of AEW&C tech from various sources of suppliers. At least that's my understanding from what been gather so far from all information circling around.
There is also the question of the radar itself. If it is something new, say from Aselsan, the risk and cost will be even higher... For just a few (2, 3?) platforms. Not to mention how long this will potentially take.
I don't think they want to reinventing from scratch, there are simply too expensive and you are right on that. However TNI-AU want to find system in market, integrate all available on the shelf tech and products and able to be use with various Fighters types, radars, and missiles system that TNI have or planning to procure. They already source Greece Scytalys to help them create base network. Now they seems want all systems to be able to work on that parameters.
It is harder work to be done and choose, but TNI and MinDef wants political independency to work it out on every system they have procure. That's seems why they choose this way on working. Which is also (if no other political consideration) why so far they are avoiding US base systems. US are not that open to let their systems intergrate to other systems especially if it is Rusian or Chinese.
Personally I still suspect on SAAB Erieye base on fact that in the last IndoDefence they are the only supplier that bring AEW&C. Their booth is small and basically what they are selling mostly focus on AEW&C solutions. French are also choosing Erieye and Prabowo's have strong inclined to use what Frenchie and also Turkiye practices.