Not sure how oil near the turbo indicates bottom end issue? I would be looking at oil breather, turbo seal, turbo oil supply hose, turbo oil return......
The new turbo cost sounds like they are sourcing a unit from Subaru. It can be a pain to replace dependent on how seized the nuts and bolts are. I sourced a refurb unit (with upgraded core) from fastline in Northampton for about £650 so then it is just labour and gaskets.
For a short block rebuild I would have said more like 3k but that relates to what you find once the engine is in bits and the type of items / quality / spec used in the reassembly - any engineering work done to bores, head skim etc (Subarus can go oval over time)