I wouldn't really bother trying to sell the car with a dead engine as people will value it at pretty much nothing even though it's a nice tbsti
Unless you found the right buyer which could happen but is unlikely.
So that leaves fixing and flipping or fixing and keeping?
Fixing and keeping:
As Owen said, a 2008 onwards 6 Speed STI gearbox alone will fit right in with no modifications, the post face GTBs also use the same R180 torsen rear diff as the STIs.
The cost of investigation and repair of your transmission could possibly be more than a 2nd hand box that's in good working order. It's somewhat of a gamble either way unfortunately.
The DCCD and front LSD is definitely a nice bonus, I got a GR STI (08/09) gearbox with 33,000km installed in my car by god they are hard to track down
If you have the budget go crazy and spend even more money to upgrade your rear diff and install a DCCD controller (Otherwise the power split will stay fixed at 60% rear/40% front but it is not needed to function at all)
Regarding the engine, I'd be inclined to re use what you can do it properly, new 257 bottom end, build it, get it tuned it and go.
Car will
for a long time - it's the current plan for my own car, my stock engine is still ticking along fine though fingers crossed
Fixing and flipping:
Leave gearbox alone, throw some money at it to install a 2nd hand engine then cut your losses and sell up.