there was a write up in turbo Mag about a Turbo Opel GT
1994 i think
sorry i cant remember if it was Carburated or fuel injected
the 75 Fuel injection is pretty cheap if you can find one in a junk yard
i bought an 2.0 Opel FI setup from Opel GT Source for $300
but that was a few years ago
had every thing but the header and the fuel pump
give Gil a call
As for the C20Let I think you are talking about the Opel 16 V motor from a Vectra?
(i think you have a great idea buy the way)
anyway they are high but you are mostly paying for the stuff to put it into a GT
other people put some brain power into figuring out the mod and that's what you are paying for
but there are options none are cheap
you could ask Gil if he would sell the kit to install the motor separately
and maybe give these guys a call
https://www.needforspeed.co.uk/index.htm
the guy is a racer and a few years ago seemed willing to ship my a 5 speed trans from a junk yard
his company was just starting out so he might not be willing these days
also check out the opel/vaxhual stuff
i have ordered from him and never had a problem
I even got some AVO Adjustable shocks
HTH
Davegt74