There are a lot of options questions you should ask.
For one maybe 6k for a brand new engine is Ok price I would not know, but you can definitely get a rebuilt for half the price.
Another option to rebuilt yours (if possible).
So that bring me to the questions you need to ask/answer:
What do yo mean under "blow turbo"? did the bearing actually broke apart and metal parts get sucked in the oil, or it did not go that far? For the second turbo, what turbo did they put in?
Did dealership find metal particles in the oil?
Does engine run or it seized? (If it run, it possibly could be flushed, oil coolers, collectors can be replaced)
If it run it could be rebuilt (labor intensive but still not 6K!)
How many miles are on the car? what is the general condition on it (I assume since you payed at least 9K for it, it is in good condition)
The bottom line is that there are cheaper options that 6K for the new engine.