how much Dexcool sludge is too much?

[Yon]

Went to go look at a car to buy ('07 with 3800 and 220k mi) and I was put off by the residue of Dexcool sludge in the radiator. I stopped the inspection right there, but am I being too quick to condemn? I mean, it's not caked all over, but I'm imagining how small those passageways are in the heater core and I don't want to be replacing that. I'm also wondering if this is indicates enough to cause hot spots in the engine block, etc. There's green coolant in there now. owner says he flushed out all the dex and replace with green when he did the water pump 10k miles ago. Please see picture: