hi,

...with a valuable option: remote SW updating, just to send HEX file to the customer

Well, if you do not care about that your software will not be protected from copying so this is the point.

AT89C51RE2

I think you meant AT89C51ED2. Yes, we use it here too without problems except ones indicated in its Errata.

I'll see about this extra GND, because it looks, doesn't it, as it is missing the connection with GND !

Maybe. But I still cannot understand what that phrase means. Seems we need ask somebody who uses T89C51RD2 about this pin and "Sales Office for ground connection".

Regards,
Oleg