He lives in Germany, I think TUV will ask for lots of papers, stamps and signatures how it is converted to LPG.

I see, you know Germany

And yes, in general, you are right. But that doesn't mean, that you have to make everything as it is supposed
So, first of all - and the real good thing - since the car has Euro 1 exhaust emission standard, there is no need for an emission report. That saves between 150 and 1000 Euro (the first if the manufacturer made that report already for that specific car - which is unlinkely - and the latter if TÜV makes it for you).
Second: Basically the installation in Germany is only allowed by certified person, but - thanks to EU - that certification is only valid to Germany, not to other EU countries, so that rule has no meaning in reality unless all components of the LPG system fulfill a specific safety norm (don't know the number right now) and you have a manual for it.
Therefore I will do the installation it on my own (pssst, don't tell!). So, I only will to pay for the safety gas check at TÜV and the registration of the LPG system there. That will be about 150 to 200 Euro (I will make it together with main inspection in February). And for the LPG system. That will be about 500 Euro. So, all togther let's say 700 Euro.
It would cost about 2000 - 2500 Euro when done officially in Germany and about 1500 - 2000 when done in Poland.
However raises a few questions:
How many KM did your engine run?
How old is the car itself?
As in, is it financially still attractive to convert this car?
The engine has 190.000 km, the car is 21 years old.
And yes, it's attractive since I will drive to Greece in November or December, that will be about 6000 km. That will cost about 750 Euro for gasoline (1.40 Euro paying in average for one liter, 9 liter/100 km). If I drive that distance with LPG it will cost me about 450 Euro (10% more mileage, 0.75 Euro) So, I will save 300 Euro only on this trip! And minimum 30 Euro every week when in Germany. So after 13 weeks (or let's say 3 months) at the latest, the LPG system is paid

And even if the car dies in the next 3 months, then I just swap the LPG system to another one

So, all well thought. And i got the informations I need, the car will get the conversion in the next weeks as soon as possible.