mate take the option of the $1500 R33 GTR bottom end that the guy replied in this thread.
add another $1K for your mechanic to do the job, you are out of pocket by $2.5K. not $5K for a wrecker motor. or $6K min from "that company name you mentioned before which I can get someone to tell you stories about"...
If you plan to keep car, then while the motor is out, do some or all the oil mods that are very well documented in this forum (search for the words "SYDNEYKID+OIL+CONTROL" to find a little thread about it)
depending on how many of them you do, the cost will be small or huge.
If I were you I would..
buy said R33 GTR bottom end.
while the engine is out, order some oil restrictors, put them in, fit the back of head drain, and the R33 bottom end has the better crank collar etc..
put it all back together and sell it.
at least then you can say that you are selling a decent car which has had some of the oil control mods done and with receipts.
with used cars, it's been said again and again.
Buyer beware.
The seller did what he had to do to sell his car.
the price was too good to be true and always is.
the fact is, once fixed like I said above, you still have a GTR that will be OK and has set you back a total of $15K max.