The O 405 / O 405 G was produced over a long time frame, which means that technical specifications changed many times during the years.
The earliest O 405 / O 405 G buses were fitted with the OM 427 h (150 KW or 177 kW) naturally aspirated engine or OM 427 hA (206 kW) turbocharged engine and Mercedes-Benz W 3 E 110R or W 4 E 110R gearboxes.
In 1987, the first update saw the introduction of the OM 447 h (150 kW and 177 kW) naturally aspirated engine and OM 447 hA (223 kW) turbocharged engine coupled to the same gearboxes.
In 1989, the first CNG engine was released, model M 447 hG (150 kW).
In 1990, the OM 447 h engines were improved slightly to 157 kW and 184 KW.
Mercedes-Benz discontinued production of their own gearboxes in around 1992 so ZF and Voith gearboxes were used instead from that year.
In 1993, the Euro I engines were released, with the turbocharged OM 447 hA (184 KW) or turbocharged and intercooled OM 447 hLA ( 220 kW).
In 1994, the Euro II CNG engine was released, model M 447 hG (175 kW).
In 1995, the Euro II turbocharged and intercooled OM 447 hLA (157 kW, 184 kW, 220 kW) engines were released.
Notably, many other specifications changed such as door styles and the optionally available VDV driver's area after 1995.
Hope that helps.