My input for 400bhp:
Opt for no less than a GT3071r 63ar and 830cc injectors. I'm not so familiar with UK tuners, as I haven't lived in Europe long. However, Eurodyne (from Canada) has been having excellent success in North America, with their Maestro tuning suite program.
VW was screwy with how many different cylinder heads they released for the 1.8T over the years, AGU and AEB cylinder heads being the only "large-port" cylinder heads. I'm actually looking for one of the two to further progress with my own build. For a turbo that pushes as much as the GT3071r, I strongly recommend obtaining one of the larger cylinder heads, if you do not already have one. In conjunction with the cylinder head, I also strongly recommend a SEM intake manifold with 70mm throttle-body, as it is proper in supporting the flow of the GT3071r.
The factory crank is sturdy enough. No worries about breaking that. Connecting rods, on the other hand, are essential and easy to swap out...as well as pistons. To sum it up: drop the oil pan, remove the cylinder head, undo the connecting rod fasteners, push the piston and connecting rods up through the top of the block, carefully re-hone the cylinder walls, properly gap the piston rings, assemble new rods and pistons, drop assembled units down into the cylinders, check clearances and reassemble the motor. It's more in-depth than that, but it's incredibly easy. It only requires attention to detail.
Regarding the transmission, performance differentials are awesome and I love my Quaife ATB, but it's not essential. Drive-shafts and clutches, on the other hand, will fail if you're consistently giving it the beans.
Earlier, Bill, from Badger 5 was mentioned. He's a good guy and knows a thing or two.
In all, I'm interested to see how your project goes.