Garages like to charge lots on this! 4k is -ridiculous-, 6k is unbelievable!!
The engine is just a 'redtop' with lower compression - pistons, manifolds, the turbo and the bits (obviously) and different ECU.
It'd be cheaper to do it this way, as the engines have skyrocketed to over 1K, I remember when they were going for 400 - 500 quid, and that was only 3 years ago. A redtop costs around 250 - 350 quid. Get the engine into a tuners, buy some forged turbo pistons and rods. And if you're really tight, get a turbo unit from a diesel!!
You haven't reached the price for a Calibra/Cavalier Turbo engine, even after being rebuilt.
The Corsa needs ... new mounts.
In fact, it only needs one, although the old one can be redrilled to fit. You can buy three new mounts for £££, or use your old ones and a drill.
It also needs new driveshafts, as the new gearbox won't match the hubs (larger engines have more splines on the driveshaft stub).
Or, changing the whole strut for a better type means you can get the driveshaft from an Astra, Cavalier or Calibra for 10 quid.
And MTech charge over 100 quid for a 'special' driveshaft!
You will need better brakes, which comes replacing the old whole strut for a 25 quid Calibra/Cavalier/Astra effort (again, bypassing the need for 200 quid brake setup), and a decent servo/master cylinder while you're at it - 10 quid from a scrap Omega V6.
You don't need new rear brakes, drums do the job, discs are pretty much cosmetic on the Corsa.
Finally, you need a different thinner drive belt (it rubs on the side of the engine bay) and an angle grinder to take 1cm out to fit the alternator properly.
There's a decent intercooler on the Frontera. One cost me 20 quid from Green Lane salvage!
There's a slight problem with the turbo Corsas, and that's traction, you will never use 1st gear!!