From the rules in 5e, I understand that a combat encounter starts with the DM determining surprise, then setting starting positions, and rolling initiative. Then round 1 starts. Surprised participants can't perform any actions or bonus actions on their first turn. They would, of course, have the ability to take reactions after the end of their individual turns in the first round.

When one side has foreknowledge of the enemy through divination, good spying, scouting, or other means, and wants to use that to set up an ambush, has the combat already started? Should initiative be rolled now? Are they forced to act at this point if they want to exploit the surprise condition (for example, to use the Assassinate action)?

For me it makes sense that the combat starts then, and also to roll initiative, since many participants start doing combat actions like spell buffing. But I see no reason why the surprise condition should end then. The way I see, it the party could prepare for an unlimited number of rounds before the other side becomes aware of a threat. Is the surprise condition maintained until then?