Don’t get too anxious about this. I’ve done it twice, and it’s not all that difficult. I would recommend removing the flange with the distributor attached. Before doing it, mark the direction in which the rotor is pointing; make a mark on the edge of the distributor body. Having said all this, from the moment of removing the flange/distributor, do not move the vehicle with it in gear or turn the engine over. This is because when you put things back together you want the rotor pointing to the very same spot. That way, there’s no change to the timing.
I’ve avoided using gasket sealant on this installation. I’ve only used a light coating of oil on both sides of the gasket so it doesn’t go on dry.
You mention you have purchased two of the distributor shaft O-rings. The moment you pull the distributor out to change/renew that O-ring, you will need to set the timing again. If the leak is not coming from that area, you might do well to leave that repair until later.
If you insist on pulling the distributor from the flange, then, yes set the engine to TDC. When you put everything back together you can set the distributor with the rotor pointing towards the #1 plug position. Usually there’s a small notch/mark engraved in the edge of the distributor body.