There are so many tea estates that it's not easy to determine which are the best for visiting. Some are more receptive to visitors than others.
One of the most frequently visited estates is Madabpore Tea Gardens. It has a lake on the premises that hasn't been treated with the respect that its scenic potential deserves, but is pleasant nonetheless. The turn-off for the gardens is about 1.75 km beyond the tea Resort compound.
The sprawling Finlays estate is reportedly not so hospitable. Politely pleading ignorance and leaving should be enough to deal with estate managers.
Sadly, tea-estate managers aren't overly interested in providing accommodation for travelers, though one suspects that the first to do won't be the last. However, all estates have guesthouse facilities for friends, family and visiting VIPs; given Bangladeshi hospitality you may be lucky enough to be considered within one, if not all, categories.