I need your honest opinion

What I understand from your initial question is what point do you stop putting in more money than the car is actually worth.
I have no knowledge of what the market is like in your area, but I don't think it is going to be much different than the market in North America.
4 door cars are generally always less valuable than a comparable 2 door, whether it is because of styling or simply that the "performance" version here were almost always 2 door models, unlike other parts of the world.
Should you sell it for something else that you are interested in? Well that is a good question, but only you can answer that one. I personally believe there is a point at which I can no longer continue to put more time and money into any vehicle that I own, simply because it is no longer economically sensible. Having said that, I also do most of my own work and have been for nearly 50 years now because I can't afford to pay someone else, and I don't generally trust most people in the automotive industry.
What you described as the condition of your car is exactly why I don't trust people. It appears that you have been sold a dream wrapped in a pile of ****.
You were looking for an affordable classic car, and you were grossly taken advantage of by someone looking for a fast buck. It is an unfortunate but very real problem in the world of classic cars, you were not the first and certainly won't be the last one to be screwed on an old car.
I would cut my losses and get out while you can, and hope that the next one is going to be a better fit.
There is no such thing as a classic car that doesn't require some amount of work or money to keep it reliable and safe, but I have to believe that you can find a much better starting point.
Just my .02.