Nobody would say "a big and old office" in english. Yes, that is the literal translation of the arabic because that is how adjective lists are formed in Arabic, but it's ridiculous to insist on the "and" in English
Why is there an 'un' after 'kabiir' but not after 'qadiim'? Does only the first of a list of adjectives require the 'un' ( sign of indefiniteness )? I've noticed that this seems pretty consistent throughout the course ( if the case endings are used at all! )
It's not technically wrong, but this is not how English is spoken. In spoken English, the phrase would be [article] + [adjectives] + [noun] i.e. "a big old office". In spoken English, the conjunction وَ would not be necessary and would not be used.