Actually, I believe the biggest obstacle to a deal is still the avionics and radar, but with a twist.
But first I think the record should be set straight here with regards to Russia only offering 'junk' to China before, that's mostly Indian wishful thinking, the Su27SK and Su30MKK were about as good as what Russia had available at the time.
The big difference between India and China was that China preferred to buy off-the-shelf and consequently got their kit years or even over a decade before the Indians for a fraction of the price.
China did also experiment with paying Russia to develop more advanced versions, which is where the MK2 and MK3 programmes came from. But both programmes were terminated after the first batch of 24 Mk2s were delivered because of a number of factors, the biggest and most important was the quantum leap the Chinese indigenous weapons industry went through in that time.
By the time the MK2s were delivered, Chinese domestic radar, avionics and weapons technologies all started to rival if not outstrip what the Russians could offer or even promise. At the same time, the balance of power in the Taiwan Strait had already tipped decisively in Beijing's favour.
As such, there was no real need to waste so much money on imported Russian kit that was noticably less advanced and capable than domestic offerings.
Today, that dynamic has not changed where it comes to radar, avionics and weapons, so the situation has actually been turned on its head. Rather than wanting Russia to offer better and more advanced radar and avionics, I believe the PLAAF would prefer they offered none.
No matter the actual number of Su35 sold, the different radar and avionics (I am assuming the Russians are not going to allow the Chinese to have the full source codes so they would integrate what they want to those Su35s) will create huge headaches for the PLAAF in terms of being able to seamlessly integrate the Russian kit with their own, as well as logistical problems in terms of spare parts and weapons. The Russian radar and avionics would also greatly complicate training.
It just makes no sense for the PLAAF to pay more than what their domestic supplies are charging for, for at best a marginally more capable product, that will soon be totally outclassed by the J20, while creating all those operational, logistical and training headaches for itself.
If Russia offered a stripped down Su35 with no radar or avionics and just the airframe and engines, I think the PLAAF might take the deal so long as the price is reasonable.
They could just install their own radar and avionics so the Su35 would be fully comparable with Chinese weapons, other Chinese military assets like AWACS, warships etc and not create too much of a logistical burden, especially if part of the deal is large scale 177S engine imports to be used to upgrade other aircraft from the J20 to potential up-engined versions of the J15, J16, J11 and even J10B.
But again, a big part of that assumption is the WS15 won't be available until 2018-20 and only in limited numbers even then.
However, given the news about the majorly updated WS10 emerging from Zhuhai, even the 117S engines might not hold much attraction for China.
So the more you look at it, the less sense it would make for China to go for the Su35.
