Sorry I do not speak Deutsch. I hope you can read my English or use Google Translate.
If you are considering buying a GTO (which are all RHD!!) it seems to me you are making a compromise that you will regret and that the cost of making it meet local registration laws could be very expensive and better spent buying a nicer car that you can be proud of showing off. You are looking at the 3000GT because it is your dream car and, like me, you are at an age where you finally can to fulfil that dream. Right now I think you are too emotionally involved and are going to end up buying the first car you find.
Since it is your dream car you should start with a clean sheet of paper, write down all the features you want on the car, and then take the time and effort to find THAT car. This is exactly what I did and after some effort, which was half the fun, I found the exact car on my list. Remember different years had different features as did cars in different markets. 94 was the pinnacle of the design from the standpoint of the most number of OEM features and upgrades, also the only year with glass projector headlights in markets that got them. Since you appear to be able to import US cars don't be afraid to look at those too since they had more features available than any other market. Green cars are not hard to find over here (mine is

You will have some choices to make though. For instance, you like pop up head lights and 6 speed transmissions but they were never both available on the same car from the factory. If you want to end up with both you would have to buy one and swap in the other. If you are looking to increase horse power in the future you should probably buy a 94 or newer since those engines had 4 bolt mains instead of two, a stronger transfer case, forged instead of cast crank, and 20 more horsepower. Don't buy a non turbo model with the plan to turbo charge it later when you have more money. Normally aspirated engines have higher compression and adding turbos requires an expensive engine rebuild with new pistons and rods plus all new wire harness and ECUs. It has been done but you are much better off starting with a car with factory turbos.
Cars with power upgrades can be good deals (mine was) but you have to be extra careful since that increases the chance they were driven very hard. Also find out when the last 60,000 mile service was done. This is very critical because these are interference engines and if it is past due you can do serious engine damage plus it will cost you about 740 € to have it done, possibly more depending on the cost of parts over there.
If you have questions about US cars I would be happy to help. I have also been buying used parts for these cars from around the world so I also know a fair amount about UK spec cars, GTOs and GTO MRs. If you decide to import I can refer you to several companies that specialize in that. Good luck on your search and I hope you take your time and choose wisely.
Mark