I've just finished fitting a new turbo to my parker 6.3 today. The wastegate had indeed seized and the whole unit was very corroded and therefore could not be reconditioned. I purchased a new turbo via universal turbo's in Sandleheath who were really helpful and knew their stuff. They soaked the old unit in an oil bath and managed to remove the manifold so that saved a few pennies. The new turbo was £795+Vat. When i originally stripped the turbo i had to rip the turbo blanket which had welded itself to the Turbo. I would have probably tried to salvage the old turbo blanket if i had known how expensive a new one was. £300 inc for the turbo blanket.
I found it a very fiddly job, the only advice i could offer would be to go careful when tightening up the bolts on the new unit as they are only threaded into an aluminium block and if i did the same job again I’d invest in a flexible extension to get to the awkward jubilee clips. Hope this helps.