>What was the state of Serb forces in target areas
It was organized okay for the numbers they had, but when you look at the target area, you could immediately see they couldn't really contest with the Croatian army. The whole area was way too spread out and to make it worse, not very wide, so no defense in depth was possible. Croat forces could attack areas and cut them off with impunity.
>What sort of prep time did Serbs have prior to atrack (in terms of months)
They knew it was coming. Of course some were still hoping the UNPROFOR to stop the Croats, but they were few. The Croats never really gave up attacking the area trying to take it back, so only a naive person would believe there wasn't going to be an attack eventually. As for actual prep time for the operation, who knows. A few months earlier Operation Flash should have given them a pretty good indication of the threat and of their own weakness.
>Was there any chance to restucture, re-equip and re-train Serbian forces in the region, the way Croats did prior to Storm
No, they lacked the manpower, they had no economy (it was all dependent on Yugoslavia), no resources, nothing.
>If given proper retraining and all, could Serbs stand a chance/force a deadlock with 130.000 vs 30.000 force ratio
No, what they could have MAYBE done is delay them long enough for the Yugoslav army to reenter the war on their side (which wasn't going to happen). It shows in the fact they chose to use ballistic missiles on Zagreb just because they couldn't really do anything effective on the frontline.
One thing to note here when dealing with Serbs, they have an out-of-touch view with reality and will claim they could have done this or that, they beat NATO, greatest warriors, and so on. A lot of them have problems facing up to and living in reality (I guess because of their history) and that is what killed them in the Yugo wars in my opinion.