connect the coolant pipes up, pressurise the cooling system with a bike pump or air-compressor (carefully!) and listen/look for leaks. If you do this with the sump off, look at the oil-drains from the head (failed H/G?).
edit: That white stuff does suggest it's been run briefly with water-in-oil (prev. owner?). Have you followed the oil and cooling lines from turbo to block? It does suck that you've found this out after all the work so far.
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