W126 500sel and sec Consumption

Hello!!!!!!!!!! I would like to buy a 500sel or sec, from year 09.1987 with G-kat 185 Kw. So that the maintenance costs are affordable I want to convert it to LPG gas and it should reach the pollutant class D3. I have now heard that a car with car gas and Euro 2 reaches the pollutant class D3. Does anyone know if that’s true? Then also the approximately 335€ tax for such a fun OK. Who knows where you can convert to gas and Euro 2 or D3 cheaply. And what is interesting to me? How much does the 500sel and sec consume with normal fuel? MfG Wolk7777