I would like to thank everyone who took the time to reply to this thread.
Your input is valuable and a great help for my decision.
So far, from what I understand, the opinions are mixed and there are many approaches to such a build.
I have clarified with local MOT what is acceptable and what not and the situation is as follows:
I can build a convertible but not a coupe. Any car that is registered as a convertible has to stay this way.
I can do whatever I want with wheels, brakes, lights, mirrors and exterior "but" I can not change the dimensions of the body front to back and left to right. (A little bit debatable this one as long as it doesn't exceed 5-7 inches).
I can swap the interior and make alterations even the dashboard as long as I keep the functionaliy and -here is the trick- I can skip the airbag as long as there is no light flashing for it. So full mod of the dash is an option.
Having talked with two sellers, one that just sells just the shell and another promising to deliver the full package including wheels, bumpers, LHD dash, seats, side fins and all other bits, I think it is better for me to arrange a full deal with him and try to smuggle somehow two large body panels and two very big boxes to France and then ship within EU.
Asked if there is an option for UK to send things abroad without tax (we do this a lot) and the recepient pays local taxes but it was a dead end.
So... we are talking for a total of € 9.500 for all that (without speedo, tacho, interior trim etc) and on top of that I need to export, transfer, import.
Further to that I have to fabricate door panels, a console, find a steering wheel, door handles interior exterior, upholster the seats and cabin, dash, doors that I estimated at € 4.000 locally and another € 2.000 at least for bits and bobs that will surely be needed.
Adding a full powder coating of Z3 parts (because island) the Z3 itself and finally prep and paint I have to spend another 16 to 18.000 (for a 2.8 or a 3.0) leading me to a total of 32/33.000€ not having to pay taxes and assuming transport will be no more than 1.500/2.000 for insured items, to end with a car that 1-nobody will ever want to buy locally, 2-will take about a year or more to build and 3-requires some more money while being built for the "unexpected" bumps that a fiberglass fitment can have.
So I end with a starting sum of 35.000€ that will surely be more.
Tough call. We are talking 911 money here and maybe Ferrari teritory.
So... for those who answered and for those who built one.
Was it worthed? Is it what you really thought it would be?
Thank you.