To determine the number of differently named lands you control, count each land you control once, but only if its English name isn’t exactly the same as another land you’ve already counted this way.

Use the sacrificed creature’s power as it last existed on the battlefield to determine how many cards to draw with Susur Secundi’s last ability.

Once you announce you’re activating an ability, no player may take actions until after the ability has been paid for. Notably, opponents can’t try to remove lands you control from the battlefield at that time.

Once you determine the cost to activate Survey Mechan’s last ability, you may activate mana abilities to pay that cost. If the number of differently named lands you control changes while activating mana abilities (probably because you sacrificed one or more lands), the cost to activate the ability remains what you previously determined.