From here you don't have to pay anything... I do not know from Russia.
I think 16-20 are HUGE tires for a 421.Prompt please the size of a tire on Unimog 421.
At me stand 16-20, at turning movement they touch absorbers.
Like MOGME said, if you can find some 11x20 rims, you should be able to find a lot of tires that fit it pretty cheap. I bought 7 "new" 14.5R20's for about $140 each.I think 16-20 are HUGE tires for a 421.
Usually MB recommends 10.5-18, 10.5-20 and 12,5-18/20. Some people will use 20 rims (406/416) in a 421 .....
My guess is that you will need spacers if you want to keep those tires.
Maybe someone from the forum can post pictures of spacers I do not have any.